@Moonlion - Lots of great excursions, so it was hard to decide for certain ports. First, it all depends how much walking and/or length of excursion you would like. Second, if tours appear sold out at booking time, once on board talk to guest services and get added to the waitlist. This worked for us in Holyhead & Dover.
As for private tours, we originally booked one for Invergordon, but the port was cancelled a few weeks prior to sailing and were fortunate to receive a refund. The other private tour was in Shetland, as we wanted a full day tour.
Edinburgh - we took the included tour and it was probably our least favorite. The guide was fine, but the tour group was too large for a walking tour in and around Edinburgh Castle. Regardless, well worth taking the tour for an overview of the city.
We also took the included tour in Belfast, Dublin, Liverpool and Greenwich. In all except Greenwich, we stayed in town and took the provided shuttle back to the ship. In Belfast after the included tour did the Titanic Museum. Interesting museum about more than just the Titanic.
Optional excursions:
We selected Orkney Wartime Legacy, as the Orkney Stone Age was sold out. We thought about a private guide, but most required a non-refundable deposit. The Orkney Wartime Legacy was small mini bus (24 passengers) tour. Wonderful guide who was a naval historian, but also provided some information about the Ring of Brodgar.
Holyhead - Ffestiniog Railway Tour. It was a long day, but well worth it! Our guide kept us on time and even added an additional stop at Conwy Castle before returning to the ship. Lunch included at Betws-y-Coed.
Dover - Canterbury Cathedral and LeedsCastle. Another full day excursion, but well worth. We went to Leeds Castle first with guided tour and then to Canterbury for DIY lunch and then tour with guide inside the Cathedral.
Enjoy your cruise next year!