@Pepijn Berlin has fantastic public transport, so why feel restricted to the- very crowded and very busy!- area around Hauptbahnhof? I can try and connect you with my daughter and/or son- they both live in Berlin. Maybe they can come up with an idea. BTW: we use Servas.org a lot when travelling. It's a great way to meet people, and we have just had guests from Berlin/Pankow who were very nice :)