My daughter & I were on the Constellation Baltics cruise in August 2003. FABULOUS cruise. We used Red October in St. Petersburg, and booked a guide for Berlin while at home via the internet (Nick Gay's Original Berlin Walks). Tom Abbott was our guide. He met us at the train station in Berlin, and we walked for 3+ hours with him as our historian/guide. Fascinating! The train ride (which you can also book on your own when you get there) from Warnemunde (the port) into Berlin was 3 hours, but well worth the time. The rest of the ports can be done on your own. I found Rick Steve's book on Scandinavia very helpful.
The Constellation is a lovely ship and the crew is great! Enjoy your cruise!
We are going back to Europe this coming summer on the Holland America Westerdam (new ship), because we've never done HAL and the itinerary sounds very interesting