I was in Washington D.C. a few weeks ago for work but managed to tag on some sightseeing over the weekend.
If you go I'd recommend doing a night tour of the monuments (I was on my own so walking around at night wouldn't have been the best idea). My other favourite places were Arlington Cemetery which is beautiful and moving, the Air and Space museum and the National Gallery (which has an amazing joining tunnel and cafe). You also have to stop by Astro Doughnuts.
I liked pretty much everything I saw to be fair - except the weather which was very British and drizzly. It's like walking around a movie set at times, so many of the sights are on our screens so often.