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. After 17:00 the entrance is only € 2.50 for everyone and after 19:00 you don’t have to pay anything! If you just want to relax without lounging at the beach you can access the upper terrace generally for free. Even in autumn and winter Weißensee is a really nice place to have some hot drinks while watching the sunset or enjoying the sky.