The best thing to do on a hot summer afternoon in the city is to spend it with friends in a roof top bar. They have the best view over the city and the sun hits until last minute. Our favourite Berlin rooftop bars that we can highly recommend are Klunkerkranich in Neukölln and Deck5 in Prenzlauer Berg.
Klunkerkranich is located on top of a multi-story car park right at U-Rathaus Neukölln and offers a very typical Berlin atmosphere. The afternoons are relaxed, you will even see families with kids, then in the evenings it turns into a cool little club.
Deck5 is a really relaxing beach-sand-rooftop located at metro station Schönhauser Allee, that even offers BBQ on sunny days.