The beach at the lake Weißer See in the district Weißensee seems to be one of the last oases in the concrete jungle of Berlin and is situated only 20 minutes by tram M4 from Alexanderplatz.
It looks like a park from the outside, and as soon as you enter the green forest you will find a circular lake, which seems totally isolated from the outside world. Relaxing is allowed along the whole bank but the Strandbad (Engl. lido) on the eastern side offers some extras like a sandy beach, sun loungers, a bar, hot drinks, snacks, and BBQ.
The Strandbad is open daily and the entrance is free if it’s not swimming season. In spring, the Weiße See is a really nice place to have some hot drinks while watching the sunset or enjoying the sky.