How fun!
I think the best things I saw in D.C. were the Smithsonian, the Holocaust Museum and the Mall, with all the memorials, Lincoln, Washington, Korea, Vietnam. Oh, then there is the Supreme Court, the Library of Congress, and Arlington! If you want to see the US Capitol, you need to book a tour in advance - either online or through your congressmen.
There is so much to see and do! You will want to go back just to see the rest of it!