The Liepnitzsee is really worth a trip in hot weather. It is a large, idyllic and very clean swimming lake surrounded by forest areas. It is located far north of Berlin, which is why it is not so overcrowded. The best way to get out is by S-Bahn (for example S-Bahn station Bernau) and then along nicely developed bike paths, mostly in the shade of the forest.
There is an island that can be reached by ferry (or by swimming), there is also a garden restaurant, the Insulaner Klause.
These asphalt bike paths were created especially for the forest settlement during the GDR era, so that the government of the GDR could comfortably cycle from there to the government …
Very nice lake and water. Unfortunately, the lake degenerates due to the many daily bath guests, who are not able to take their whole rubbish back with them :-(