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.
There's nothing like strolling around the Mall when you're in a pensive mood. With the White House to the north, the Capitol to the east, and the Supreme Court just beyond that, you are truly in the center of the western world. It's a very powerful feeling.