If you ever want to take a walk around a lake and you only have an afternoon but no car: Visit the Schalchtensee! It is a gorgeous lake with a wonderful walk around path. It takes you about 20 min by train from Berlin's city center! The station (S1.) you have to get out is called the Schlachtensee. If the weather is great you will not be the only one, but on weekdays you don't see many humans.
I know many humans who actually go there for a swim. The water can't be that bad. I have seen many little access points to the water and a few beaches... Miss Stanhopes just dips her feet into the water, that it.
Sunsets there are, in my fluffy opinion, absolutely gorgeous!