In the summer season, you can go cruising in Spree river around all over Berlin with a one way trip (2 hours) or a round trip (4 hours). You'll be able to see the most beautiful and iconic monuments of Berlin without having to walk through the city. This trip is highly recommended if you are new in Berlin.
Spree river runs through Germany and Czech Republic over 400 kilometers. But in Berlin it's a main attraction because it offers you a 4 hour round trip arround all Berlin from east to west. This trip is the best way to see the city without having to walk. I highly recommend it.