Near the Brandenburger Tor you will see a Starbucks. From there the Free Berlin Tours take place. http://www.newberlintours.com/ They also offer a few others. I took that and the alternative Berlin tour which was great.
The Reichstag. One of my favorite travel memories took place here in November, 2003. Nowadays you have to register in advance to visit this historic building. I've made my registration and even reserved a time for dinner at the top.