K.M.
You may want to also go and enjoy the National Zoo! I also love going to the museums and my favorite is the Air and Space Museum as well as the Museum of Natural History. Don't forget the ever present Vietnam Memorial (you will never forget the feeling/impact it has on you for such a "simple" memorial) as well as the World War II Memorial and the Holicoust Museum (another unforgetable experience)!
There are some great places, night spots, in Clarendon which is very near Arlington where you are staying.
Enjoy your visit and let us know how it went when you return!
SIncerely,
K. M
WAHM of Chris, 25 and Brad, 17