My wife and two daughters ages 25 and 22 just returned from a really outstanding trip on the NCL Star to the Baltic Sea. The ship was in great shape, the food was good and the nightly entertainment was some of the best we have seen on a cruise - a nice mix of music and interesting acts. My wife and I had a balcony room and the girls were right across the hall in an inside room.
We booked our excursions through SPR with tours in Berlin (don't miss this, the bus ride is not that bad, great time to nap) Saint Petersburg's (Wow - no wonder it was chosen the top European destination of 2015) and Stockholm (Vasa Museum - a window into life on a war ship in 1628 with the actual ship on display - absolutely fascinating story!)
We did a free walking tour in Tallinn and the hop-on-hop off in Helsinki.