I've done several ferry trips in this area although no cruise (yet). The Baltic Sea has lots of small islands, great scenery, and sometimes dramatic weather. If the ship docks anywhere near the ferry terminals, it will be a short walk to town in Helsinki, Tallinn, Mariehamn and Copenhagen. I think Stockholm center is close also. Actually I've been in Mariehamn and I don't remember much of a town. Should be good walks there however.
I have always enjoyed visiting Helsinki. Plenty to do near the port and other interesting things are a bus or tram ride away.
Warenmunde seems to offer excursions to Berlin, but Berlin is a considerable distance away. I would do something closer.