At 4km long and 2.5km wide, the Müggelsee is Berlin's largest lake. Hemmed in by forest on two sides, it's hugely popular for swimming and boating on hot summer days. It's easily reached by public transport in less than an hour from central Berlin.
The Müggelpark on its north shore has restaurants and beer gardens as well as the landing docks of Reederei Kutzker, which runs lake tours several times daily.
For a walk in the woods, head to the other side of the Spree via the nearby Spreetunnel. Seebad Friedrichshagen is the closest public lake beach and has boat rental; it's about 300m west of the Müggelpark.