We arrived on a Sunday and not much is open. I don't know what excursions are available as we generally don't take them, but if you walk in don't expect to see much.
Wernemunde is the port for Berlin. I would not recommend taking the trip there though. It is a long journey there and you would only see a snippet of the city. It is best seen on a separate trip so you can explore the city over a few days or even a week. I would take the train to see Rostock instead. It only costs a few Euros and is a very interesting medieval city.
Helsinki is also easily explored inexpensively by public transit. Purchase a day pass for 8 Euros from the bus driver at the stop right next to the port. I recommend printing out a transit map before arriving. It makes it easier to plan where you want to go without getting on the wrong bus or tram (like we did).
Oslo was a great port of call with lots to see and do. The best way to see the city is on public transit with a one day pass and museum entrance ticket. It is 216 Norwegian Kroner ($38) per person and will get you into everything you could possibly want to see.
Must see but incredibly busy. Make sure to see the gold room.
Stockholm is another great port of call. Walking to town is not obtrusive; it was only marred by President Obama's visit causing sections of the city to be closed off depending on where his motorcade was. My wife mentioned that she would like to come back and really see the city.
Tallinn is also a interesting place to see with things off the beaten path to explore and see. We walked to the old abandoned prison and for 2 Euros you can see what life was like in a Soviet prison. Glad I was on this side of the iron curtain. We also went to the Seaplane museum with the only WWII British built sub I have ever seen. The old town is very attractive, my wife enjoyed exploring it.