Under normal circumstances, I'm not the type to take tour busses, but the double-decker tour bus we took in Berlin was great. It really hit almost every spot, and then some. I recommend doing this on your first day, hopping on and off all over town, and then figure out the other places you want to visit after that.

Sachsenhausen Concentration Camp isn't far out of the city, and easy to get to by train. Be careful, they have odd hours on some days.

Mustafa's doner kebab was awesome, and right near a subway station. Walk up the steps and there it was. It was right next to a convenience store, too. Something about riding back to the hotel with a doner and a couple beers, late at night, was a lot of fun.

We ate at a nicer restaurant called Marjellchen. I don't have much to compare it to, but we thought the food was awesome and the people were extremely friendly. It was a short walk from a subway station.

Did you take one of the city tour buses (route 100/200) or a private hop-on/hop-off?

If you get the chance I would visit the Holocaust Memorial very moving.