There is SO much to see in Berlin that it's overwhelming. I love this city. You could be here a month and not see everything. The people are great, and it's easy to get around; I've taken the U-Bahn and S-Bahn and have seen countless interesting sites. I think I've taken around 450 pictures. Must sort through all those when I get home...
At the Deutsches Historisches Museum, we were in the 20th century section and this wonderful German man came up to us and started talking all about the years of the Weimar Republic, the Third Reich, and the DDR (in English). We spoke to him for about half an hour and he promised to send me some photocopies of original German newspapers that his parents saved from the '30s and '40s, if I'd send him a book from the United States about the development of the American Constitution!
(As an aside -- we've noticed that it must be the German thing not to look at the camera.)
I was happy to see that just across the street, in the yard of Humboldt University, there was a book sale going on that same day.