Müggelsee - both of them (Gross and Klein) - are two lakes to the south east of the city of Berlin. There are some great walks to be done along the wooded shoreline where you'll occasionally stumble across a beautiful beach. In fact the whole area gives you the impression that you are far away from the city on a relaxing holiday, perfect. And of course it is free!
What's more it is a great cycle ride from the city centre and from Treptow onwards nearly all of the route can be cycled off road. Although still a part of the city of Berlin you'll find yourself in the small town of Köpenick which is well worth a visit in itself. And if you don't fancy a bike ride, it's easily accessible by bus or s-bahn too
Google map: bit.ly/OkuskA