In addition to the places mentioned by oatw13, you might take a look at the Albert Einstein Memorial near the Viet Nam Memorial and a wonderful sculpture called "the awakening" on Haines POint just south of the Jefferson Monument. Both these places are on the list of what to see when you think you've seen it all. both are outside and free.
I lived in Washington for four years and took visitors to both of these places many times.
Originally Posted by oatw13
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!