Berlin is one of the coolest, kinkiest, and cheapest cities I have ever been to. The peope might seem cold at first but are friendly and interesting. I had no problems anywhere even though I do not speak the language.
When walking through berlin the juxtoposition between the east and the west seems to smack you in the face. From the attitude of the people to the architecture it is impossible to miss. The west is modern and feels more sophisticated and honestly a little less of that cool berlin vibe. However, in the east you see the brutalist architecture everywhere and the people are way more laid back (sometimes a bit sketchy) but overall it is cheap wonderful and interesting.
Actities to do:
Teufelsberg - This is one of the coolest places. It is an abandonded NSA radar tower from the cold war that has become a mecca for graffiti artists around the world. Definitely a must see.
Kit-Kat Club - also known as the kitty by the locals. It is a german fetish club in neukolln. This is one of the craziest places I have ever been in my life. Definitely would visit again.
The Remnants of the Berlin Wall: Walk along what was once the berlin wall and check out amazing graffitti.
Beer: go into any "spetzi" or bodega and you will find the best and cheapest beer you have ever seen.
Street Art: All over Berlin there is amazing street art. My recommendation would be to go to east berlin, pick a direction and just start walking around!