Peetzsee

Near the suburb of Erkner (east of Berlin) are a couple of big and very clean lakes, Peetzsee being one of them. The problem is, that you need a bike or a car to get there, as there is no public transport nearby. If you ride by bike you have to go to Erkner with S3. Then it's a 30 minute ride to the lake. 

The beach on the pic is at the southeastern side of the lake.