There are many options at this port. The actual tourist area along the canal would occupy some. We chose a shorter train trip than Berlin. We gave ourselves a bit of time for breakfast and then took the train to Wismar. The connection is in Rostock but we chose to go back and have a relaxing beverage and snack on board instead of getting too tired. People were boarding right up to the departure time when returning from Berlin. We were again glad not to do something that long on the first day. There are a number of other towns that would be an easy train ride.
The old city is close to modern tall buildings but a world apart. We followed most of a walking tour posted by Jay Brentjes and did not feel rushed or tired. Walking by the old walls give an idea of medieval times. We had a beer at one place and then juice and a pastry on the way back. The view from the overlook is amazing--be sure to find it. You can see the old, the new, and cruise ship in the distance.
This is a nice city and maybe a time to take it easier after St. Petersburg. We took the bus into the square and wandered a bit in the market before taking the ferry to Soumenlinna. You could buy a day pass on the buss but the main ferry was not included this year. We enjoyed a leisurely stroll along the main path with only a few side stops. It was windy but not too cold and the fortress was a nice contrast to the previous days. There is more to see but relaxing was more important.
This was my favorite port. There is much to see that is close to the cruise port and transportation is easy. We saw the Vasa and the close by Historika museum for Viking artifacts. We then returned to the palace and saw the end of the changing of the guard. We saw the armory museum and St. George and the Dragon in the close church Storkyran. I would have loved to see more of the other museums at the castle and spend more time wandering the Gamla Stan. The departure time should be extended to 5pm. Two more hours would have helped. The entrance and sail away should not be missed. There are 2 hours of islands and we were lucky enough to have a warm afternoon.
Do not miss Copenhagen. There are a number of castles, churches and other sights. A canal tour was interesting even on a cooler day. We stayed a day before and day after at a hotel in Nyhavn and walked every where. It would be possible to use public transportation from the cruise terminal but we saved our walking energy for the area around the hotel while waiting for the room to be ready. Due to a cooler night and the need to get up early for the flight home,we did not go to Tivoli.