Müggelsee

That's Berlin's biggest lake, located in the district of Köpenick and about 4 kilometers  long. Except for a small path to cross the lake it is closed for motorboats. Only sailing and swimming is allowed. On the northern end there are lots of little beaches in the forest and there also is a big public open air swimming pool. The lake is quite shallow, you have to walk for a while into it to be able to swim.

The lake comes in two parts: The big one and the little one called "Kleiner Müggelsee".

On the southern shore you'll find the heavily visited Müggelsee terraces called "Rübezahl" including a big playground for the kids.

More about the lake: Wikipedia