Hey kjanina,
I’ve been living in Berlin a few years now and thought I’d also share my favorite hangout spots.
The Landwehrkanal is a canal which separates Kreuzberg and Neukölln, two interesting districts in Berlin. During the summer, you can find many people hanging out drinking beer at the waters edge with their friends. There are plenty of wonderful restaurants along the canal as well.
Continuing with the water theme, Holzmarkt is a community space run by the same people who own Katerblau, the club next door to it. It is directly on the Spree and has plenty of space to hang among the gardens overlooking the river. You can bring your own drinks but its great to support the local businesses and artists which live and work in the community by buying the food and drinks sold there.
I also love Park am Gleisdreieck. It used to be a an old train crossing/station and is now an spacious park wonderful for late afternoon picnics.
Its summer now so I could only think of things nice to do when the weather is warm. Maybe in the winter I will post again with my favorite winter spots 
…but in case you are in Berlin and don’t know what to do this (!) afternoon maybe you wanna come by 
