Germany
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.
It's a fantastic and beautiful lake in Berlin-Zehlendorf, 20 minutes from Charlottenburg. You can swim in it, and the cafe is a very trendy place that's popular with Berliners at the weekend.
U-station Schlachtensee, Am Schlachtensee, Zehlendorf, Berlin, 14163
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