Tallinin is a beautiful little town, not particularly wheelchair friendly, but doable from the ship and back. We did not go up the hill; figured that would require too much battery power.
Helsinki has an accessible shuttle bus that will take you to and from the center of town. From there, you should be able to get just about everyplace. They had a very nice flea market the day we were there. Do not know if it was a daily or weekly event.
Stockholm depends on where you dock. If you actually dock in Stockholm, they have the “painted line” on the sidewalk into town. It is quite a little jaunt, so you might want to consider a bus into the center of town.
Copenhagen is easy to get around in. Suggest you take the train to the central station and work your way back to the ship. The central station is near Tivoli Gardens and the Town Square.
I have not been to Gdansk.
If you are seeing these places on your own, I would suggest that you use Open Street Maps. They do identify many stairs in pedestrian paths that Google does not show. However, Google does offer a bicycle path in many of the “old European cities”. The bicycle paths on Open Street Maps are not that good. Also, with Google, you can get a “street view” which does help. I have found using both maps does eliminate a lot of bumps in the road.
We travel with a spare battery pack considering that we do plan some rather extensive outings. This also allows us to see quite a bit more.
Have a wonderful cruise.