what we did DIY and tour:
Stockholm: take the HO-HO that picks up right at the pier (get SEK for the Ho-Ho (Swedish $$) before you leave the US--they are not on the euro). Take the Ho-HO to downtown and walk around. You can go into the Palace, etc. but the BEST place was the Livrustkammaren. Founded in 1633, it is also Sweden's oldest museum. Set in the palace vaults, this armory isn't just about weapons but displays some of the world's most magnificent state coaches and coronation robes, even the costume worn by Gustav III at a fatal masked ball. (The king was assassinated at the 1792 ball, and the incident inspired Verdi to write his opera The Masked Ball.) They have a free kit that you can borrow for the kids--it is a box with a fake mouse and you find clues throughout the museum. My kids (9, 13) spent 2 hours doing this--super fun. They also have a good dress up place, as well. We got the combo Ho-HO ticket and also took a boat tour. We did not have time to see the Viking ship (on the HO-HO bus/boat line).
Tallin--easy to walk from pier to town and they have costumed persons selling nuts, sodas, etc.--very festive. Plus, you can do archery outside the fort with Red Riding Hood type costumed workers. Large square in center of town to get food, etc. Tallin looks like a storybook village from a fairy tale. Easy DIY.
St. P was DD9's favorite spot. We used TJ tours for a 11 hour private tour. Will use them again for 2018.
In CPN, we got the CPH card and used it for train from airport to city(easy, safe, 10 minutes), entrance to tivoli, entrance to Rosenburg Slot. We took a taxi from hotel to port for about $35. CPH is a nice, walkable city. Elaine