The way I did it was I researched first, using Rick Steves's book and travel sites. I picked out the best cruise for seeing the maximal amount of my first round ( oh cause that list just grows!). I picked a great little cruise in 2016 called the Grand Med and saw Barcelona, Aux, France, Livorno (though we chose to do Cinque Terre) , Rome, Naples, Athens, Mykonos, Istanbul, Ephesus, and Venice. We stayed in Venice 3 days, then we flew back to London and stayed there and flew home. It was aboard the Norwegian Spirit and we had a terrific time. The sail into Venice is sublime and I am glad we did it when we did.
The hardest part of any cruise is balancing the ports and the ship really is an end to t means. Long as it has beds and decent food and you are safe, that is fine. You will not spend the same amount of time onboard relaxing and partying. You get up catch your tour and are gone all day, return to eat. Maybe see a show, and sleep to repeat the next day.
Make your wish list and go... it is worth it. Then you have to start finding tours, that is the fun and sometimes expensive part!
What you do not get a on cruise is the inland parts of Europe. So we usually book a land, and then a cruise-type trip, alternating between the inland and sea. Some people are smitten with just one type, but we prefer a bit of variety. We usually plan the base adventure of 14 days and cap the beginning and end in city stays.
What we have done is very little compared to so people here and many here plan 1 to 3-month trips We sadly cannot be away that long...What I have learned is not to put a large city stay at the end of the busy cruise or land trip. Stay early before and fly home. I prefer smaller towns also so mix it up a bit!
What we have done is as follows. :
A 14-day trip through the UK, staying in London, Cotswold's, Bath, Cardiff, Conwy, and Keswick UK, and Edinburgh. It was our first and we stayed in some great B&B s and drove in the UK which is an experience in itself.
The Grand Med Cruise I mentioned above.
Rick Steves GAS tour - We flew into Frankfurt, learned German train systems, then stayed in Bacharach, Trier, Baden-Baden, and Munich Germany. Murren Switzerland, which is heaven on earth. Hallstatt, Salzburg, and Vienna. We then flew back to Paris and stayed there for 3 days.
Upcoming for us is an NCL cruise to France, Spain, and Portugal.
We also have been on several stateside cruises in that span of years since 2016 including 2 Alaskan, and two Caribbean cruises.
Hope this helps. The joy of planning is rewarded in travel and new experiences.... Good Luck!