Go see Checkpoint Charlie (lots of tourist stuff here), the Brandenburger Tor and Reichstag and Memorial to the Murdered Jews of Europe (these three are relatively nearby), Fernsehturm Berlin (needle tower), the wall (lots of interesting graffiti and memorials).. there are a lot of museums, but google them before going in. I was in the DDR museum and it was not very good, quite crowded and expensive.
There are some castles near the city (or in, like Charlottenburg Palace), you might find a trip there worthwhile. There is also a Zoo in the city.
Feel free to try the local specialty - Doner

But no, its best to avoid the fast food other than German sausages (Bratwurst, Currywurst, you should try some of these if you haven't before). And try some German beers (Paulaner, Franziskaner). Avoid restaurants where the waiters are pushy and too welcoming. I ate with a group and the waiter put a big tip on the bill, much more than 20%. We refused to pay for that, because his service was not good at all and then he started saying how this its typical for our nationalities (and we were an international group). But this is just usual big city, tourist trap stuff.
From my experience, Berlin is quite hectic with a lot of things happening everywhere. Since its a huge city, it has a dark side, too. Just don't strut around the bad part of town with a big camera bag in the middle of the night.
Hope you have good weather when you visit