--Lived there for 13 years
Like some of the others have said, you'll have to prioritize based on your interests, as you could easily spend a week in the Smithsonian complex alone
Other things to see in and around D.C.:
The Bureau of Printing and Engraving
The National Museum of the U.S. Navy
The College Park Aviation Museum
The National Geographic Headquarters
The National Gallery of Art
Mount Vernon (South of DC proper, but not too terrible of a drive)
The Goddard Space Flight Center (Northeast of DC proper, but again, not a horrible drive)