Must See & things to do @ Washington DC and area, planning a vacation?

My boyfriend and I are planning on going to Washington DC for a vacaction this summer. My boyfriend is a Veteran who is still in the National Guard and served as a Medic over in Iraq for a year in 2006. He LOVES history and has never been to DC for a sight-seeing vacation! My dad and I went there a few years ago and I remember Arlington was awesome. We also went to the Marine Sunset Parade at the Iwo Jima Memorial, which was great and something I definitely want to make sure we do while we are there. My dad and I also went to George Washington's home and took a tour of that, which was outside DC a little ways. I've been to DC twice and it is definitely a place where everytime you go back there is more and more to see. Both times I have gone, there was no planning involved, just sight-seeing on a whim. I'd like to have a little planning done this time and would like some suggestions for "Must See" places/things to do while we are there. Thanks!