For a truly impressive and very well preserved city wall within day trip range from Berlin and Hanover, I recommend Tangermünde. There is not an enormous amount to do here, but I managed to find a nice cafe for lunch and an outdoor ice cream parlor for an afternoon snack. The attraction here is the wall itself, along with the other historic buildings. In comparison to the tourist overrun old towns of West Germany, its a delight to stroll around here. Warning: Not much English spoken here.