Honestly, StrophariaCube's post is pretty much perfect.

If you go to the Reichstag, be sure to book your tour well ahead of time. I am embarrassed to admit that, though my in-laws live in Berlin, I've never been on the tour. We always forget to get the tickets far enough in advance of our visit. (I think, though, that the English-language lines are shorter than the German-language lines.)

I agree with the posters who said to skip the Checkpoint Charlie museum. There is a lot of interesting info in there, but it is presented with an obvious viewpoint and aim. If you are interested in the history of divided Germany, check out Berlin's Haus der Geschichte (House of History): http://www.hdg.de/fileadmin/static/english/berlin/

I also enjoy the boat tours, but March is probably not a good month for this.