I spent a full week in Berlin. I had thought I would do a day trip or two, but never made it out of the city.

Museum Island is the famous place for museums, but there are oodles of them in Berlin. Think about what you're interested in, and then look. For example, I went to a museum focused on Bauhaus and one that featured the work of Paul Klee.

I recommend the new museum on the site of the former SS headquarters. When I visited, it was only partially completed/open and even then it was quite impressive. Also, the Jewish museum is worth a visit. Though the Checkpoint Charlie museum is a bit touristy, it does have a lot of artifacts from that period of history--the ways people tried to escape from East Germany. I thought it was worthwhile.

The Bundestag was worth visiting, but the queue was very long, like 90 minutes in line. But that was during peak tourist season.

Great shopping. And I think more prevalence of English language than even in Munich or Frankfurt, so very easy to navigate for Americans.