Yes, thank you so much for the photos! Not only did they bring back wonderful memories, but I've enjoyed our two trips to the Baltic -- most recently my trip with my daughter and teenage son on the Norwegian Spirit in 2019 -- that I'm considering joining my sister on another in 2021! (Hence I'm back scrolling through this board!)
In addition to my thank you, I'll add my own port reviews for the benefit of anyone who might come across this thread (edited from some notes I was making for my sister's benefit):
Stockholm: It depends where you are berthed. The first time we visited Stockholm, we berthed in Nynashamn and so we joined a group shore excursion (through the same company that we were using for St. P.), and the excursion was . . . underwhelming. I enjoyed the part in the Vasa Museum and the short walk in Gamla Stan, but the rest was photo stops (at City Hall and the like) and looking out bus windows and too short for our port time. If I knew then what I know now, we probably would have taken the train from Nynashamn. The second time visited, our cruise left from Stockholm, so we had more time to spend in the city, and I fell in love with it. If you have a choice, do choose a cruise that embarks or debarks in Stockholm to give yourself more time in the city. We caught the Arlanda Express from the airport (very easy) and stayed in a hotel near the train station (also near the Icebar, where we met some of the people from our Roll Call for drinks!). It was an easy walk to Gamla Stan, where we enjoyed a wander on our first evening. The first morning, we awoke early because of the jet lag and went on a subway art tour before breakfast! (Google "Stockholm subway art tour" for advice.) We then spent most of our day on Djurgården, not only visiting the Vasa Museum (we visited early before the crowds), the ABBA Museum (which gets crowded quickly!), and Skansen (which is spread out enough that we didn't have to worry about crowds). My favourite part was actually visiting the Nordic animals in the small zoo in Skansen. The next morning, we went back to Gamla Stan and visited the palace, and my favourite part was actually the (free!) Royal Armory Museum, which it's just weapons but clothes and carriages and etc.
Helsinki: Helsinki feels somewhat underwhelming after visiting many other Baltic ports, especially if you stop here after St. P., because it's a much more modern city. The first time we visited, we joined a group shore excursion (through the same company that we were using for St. P.) and enjoyed it: we had time in Market Square, some photo stops (at Sibelius Momentum and the like) and looking out bus windows again, but we also had a guided tour in the folk museum on Seurasaari Island, which really redeemed the tour. It was a fascinating glimpse into traditional Finnish life, and the setting was so beautiful. In fact, I so fell in love with the landscape that the next time we visited, I organized our own excursion with Feel the Nature, who took us on a short hike in Nuuksio National Park. As it was the season for it, we got to pick wild berries (my favourites of which were the lingonberries and bogberries). We also got to visit the reindeer near the Haltia Nature Centre and feel them lichen! Some others on our cruise visited Porvoo, and enjoyed it. Those who stayed in Helsinki but went off on their own especially enjoyed Suomenlinna. And someday I would very much like to visit Löyly sauna.
St. Petersburg: Obviously the highlight on any Baltic cruise. This is one port where you can't just wander off on your own -- not without a Russian visa -- and for a first-time (or even second-time!) visitor, it's not really worth the hassle and expense of obtaining one and organizing your own transportation and tickets when there are so many excellent companies devoted to doing so for you! The first time we visited, we went on a standard 16-person small group tour with SPB, and we highly enjoyed it (and I also heard wonderful things about Alla, TJ, et al), but if you can afford an even smaller private tour, I highly recommend it. The smaller your group, the easier it is to get around those fearsome lines, and the greater control you have of your own itinerary and schedule. The second time we visited, we went on a private tour with Red Sun Tours, and had so much fun: not only did I get to revisit my favourites (like the fountains of Peterhof, the Hermitage Museum, and the Church on the Spilled Blood) but we got to visit more quirky attractions like the Grand Maket (I love miniatures and model trains) and the Soviet arcade games museum (which gives you some vintage 15 kopek coins to play the games!). If it had been open when we visited, I'm sure my son would have loved the Artillery Museum. For food, we visited Stolle again, of course, but then we went to try some Georgian food (that we can't get at home), and next time I want to try some Uzbek food. We would definitely do a private tour again. There's so much to see in St. Petersburg, don't feel constrained by what other people consider must-sees. (For example, I found Catherine Palace hot and crowded and the Amber Room smaller and darker than it looked on all the photographs.)
Tallinn: Oh, Tallinn is beautiful, but if you want to really appreciate the Old Town, try to avoid the crowds and start early or end late. The first time we visited, we joined a walking tour, and I was grateful our guide was so tall so that we could see her over the crowds, or we would have easily lost her. At the end of the tour, we got out of the Old Town and visited the Lennusadam Seaplane Harbour Museum, which we enjoyed. The second time, I organized a private walking+bike tour with Traveller Tours (which was not expensive) and we got started much earlier, and it was a better experience. The walk from the port into the city and even up the hill is not bad if you're able-bodied. And then, when the Old Town started to get crowded, we hopped on our bikes and rode out to the more modern outlying districts, along the harbour to where the Tallinn hipsters lived, through residential neighbourhoods (the Estonians like to put ping pong tables in their parks!) to Telliskivi Creative City and the market. We saw lots of interesting street art, and I do recommend venturing out of the Old Town to see the contrast. In the afternoon, we wandered into the Rotermann district to the Kalev chocolate shop (do eat some Estonian chocolate!) to attend a Chocolate Truffle Making workshop I'd set up with some others on our Roll Call, which was a fun social activity. If I head back to Tallinn again, I'd like to visit Kadriorg Park and the singing grounds.
Riga: We didn't visit here on our first cruise, so it was all new to us. Still, I let my son talk me into visiting the medieval castles of Turaida and Sigulda (along with Gutman's cave) with only a short walking tour of Riga. I think he was afraid that if I got to pick, we'd spend the entire day admiring Art Nouveau architecture, and he's probably right. This way, he had to tolerate only about 20 minutes of Art Nouveau architecture. While I enjoyed the castles, especially climbing the tower at Turaida, I don't know that I would go again. They aren't in the best condition (especially Sigulda), and Riga is beautiful. I did enjoy our walking tour there, and I recommend the company, Nordic Experience, that I used to organize our day. They were able to issue invoices and collect credit card payment from everyone before we left so that we didn't have to worry about payment once we were there (or no-shows driving up the cost).
Klaipeda: Another place we didn't visit on our first cruise. Klaipeda itself is small, so I chose to organize a tour (with Ulko Tours) to the Curonian Spit, my favourite place on which was the Hill of Witches. As we were a small group, we were lucky enough to arrive and be able to wander around the forest and see and photograph the sculptures before all the crowds arrived. They were swarming in as we left, and I'm sure it would have been a very different experience. (I did wish our guide had known more about the folktales the sculptures represented.) The cormorant colony was underwhelming, which was disappointing as we had some keen birders with us. We did see some bird colonies randomly later -- including more swans than I'd ever seen before -- so I'm glad we had a guide who was willing to help us find them. We did go all the way to Nida, and the amber "museum," although small, was a fun experience as we got a small presentation on Baltic amber. Word of advice: don't walk the dunes, especially on a hot day, unless you really want to feel what it might be like to be lost and struggling through the Sahara. When visiting Klaipeda town itself, make sure you search out and find the quirky sculptures, especially the very cute Golden Mouse.
Gdansk: Yet another place we didn't visit on our first cruise. Here, transportation from the port of Gdynia is an issue as traffic is notoriously bad. I organized a tour through Nordic Experience again to the former concentration camp of Stutthof before visiting Gdansk. The tour itself was excellent, although it was a difficult and emotional visit. If you'd like to visit somewhere outside of Gdansk but don't feel up to visiting Stuffhof, our fellow cruisers who visited Malbork Castle enjoyed it. Gdansk itself is lovely, if a little crowded during peak tourist season. We did make time for a photo stop at the Solidarity shipyard gates. If we'd had more time, I would have loved to stop at Oliwa Cathedral or in Sopot to see the "Crazy House." As it was, our guide and driver had to be clever to get around the traffic to get us back at the port at the arranged time!
Warnemunde/Rostock: The first time we visited, we knew that we didn't want to venture into Berlin as we were travelling with little kids, but we did venture as far as Schwerin, to see the "fairy tale" castle. It was okay. We'd joined a small-group tour (through the same company that we were using in St. P.) and travelled by train to Rostock and then on to Schwerin. My enthusiasm for the day was admittedly dampened given that it was a Sunday (so many places were closed and we were unable to visit the astronomical clock in St. Mary's church as it was closed for services) and it was drizzling rain. Warnemunde itself looked cute and I resolved to spend more time there if we returned. However, the next time we visited, our port was Rostock, which is not a passenger port, and it was Sunday, so transportation was difficult. The few taxi drivers who were there were looking for fares into Berlin, but we talked one into taking us to the Rostock Zoo, which is an excellent zoo and we spent the whole morning there. We then caught the train and bought a day pass, and went to Warnemunde . . . which was crowded. If there are enough ships that some have to port in Rostock instead of Warnemunde, Warnemunde is going to be crowded. We had lunch and wandered along the waterfront briefly before hopping the public ferry across to the Marine Science Center. The Center itself is not much to visit as it's really just some pontoons with the seals and sea lions who participate in their sensory research experiments swimming around. However, we participated in their Seal Experience, which allowed us to go out on the pontoons with one of the researchers and the seals for an hour while he explained the research that they did and how they trained the seals to participate. Yes, we got to pat the seals and even play fetch with them! (At the end of the day, we were able to take a train and a bus back to the port without difficulty. Google Maps is good with public transportation schedules.) Next time we visit, we might to the train to Bad Doberan, but I would like to visit the seals again!
Copenhagen: The first time we visited, this was our embarkation point, so we had a few days to spend in the city. My favourite museum was the National Museum with its excellent history exhibits (especially the prehistory through the Vikings galleries), my husband's and son's was the Danish War Museum. The palaces are all so different: Rosenborg is small but noted for its Treasury, Christianborg has the grand state rooms, and Amalienborg is more intimate and lived-in. Note that although you can visit all on the Copenhagen card, there may be lines at Rosenborg and you can't bypass them with the card. (Go early.) Next time I'd like to visit Frederiksborg castle. Of course, you can't visit Copenhagen and not enjoy a canal cruise. The first time we visited, we got up early from the jet lag and were the first passengers on a mostly-empty boat, which was great. Although the second time, we went in the afternoon on a hot day and got to watch all the Danes jumping in the canals and having a swim (clothing optional near Christiania). Both were fun experiences. Be aware that if you are on a canal cruise, you will only see the Little Mermaid statue from the back, and she's small. If you want to really visit her, go on foot. And if you're walking that direction, do visit the Gefion fountain too. I do recommend, if you have a choice, choosing a cruise that starts or ends in Copenhagen. (There's an easy train connection between the airport and the main station, near Tivoli). There are other places that we visited on both visits, but if you only have one day, those were my favourites (Tivoli, although it was an interesting park to walk around, had long lines for all its rides). Note that it's easy to catch the buses (HoHo or public transit) from Oceankaj, but when catching the public bus, we had to have tickets or the Copenhagen card in advance. BTW, a note on cards like the Copenhagen card: there are now apps that allow you to purchase and activate these on your cellphone (as long as you have data or WiFi), and you can put more than one card on one phone (just swipe through the cards when necessary).
Oslo: I really like Oslo for its museums, and although we had an excellent guide on our first visit, this is a very easy port to visit on your own. On our last visit, we got the Oslo card that covered museums and public transport, including the ferry across to the Bygdoy museums. The Viking Ship museum is probably the most popular (and very good, although small), but my favourite was the Fram museum and my kids like the Kon-Tiki museum best. The Folkemuseum (Cultural History) is also nearby. From there, we took public transit to the Vigeland Sculpture Park (and met some friends for dinner). Other places that we enjoyed on our previous visit were the City Hall and the Opera House (walking up the roof). Other places to visit that I know others enjoyed were the Resistance Museum in Akerhaus Fortress and the Holmenkollbakken ski jump hill.
That's a lot of words, I know, but I hope that they help someone?