Wannsee is a lake very close to Berlin and you can go by train (not really expensive). It's a lovely place where you can have a bath, walk, or take a ship which is going to take you around Postdam, the Devil Mountain or the world war two spy bridge.
Just have a look at a train map.
Updates
The "world war two spy bridge" is called "Glienicke bridge",-"Because the Soviet Union and the United States used it three times to exchange captured spies during the Cold War, the bridge was referred to as the Bridge of Spies by the media.“-wikipedia. Take the S-bahn to Wannsee station - it's then a short walk to the piers where all the boats leave from. Most run just in the summer - except for a ferry to the other side which runs all year.
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