Gdansk is definitely worth at least a day or two. Some really nice old buildings. The center was largely rebuild after the city was heavily damaged during the second world war, but the Soviets decided to preserve it as a kind of monument. The only other city I visited was Krakow. I've been told that Wroclaw and Warsaw are also worth a visit. But from what I understand all the big cities suffered heavily during the war, so if you're looking for nice architecture it's easier to find it in smaller towns like Torun and Zamosc.
Pick up some Zubrowka once you're there. It's the best vodka there is.