A mueseum, or the zoo and some shopping is about all you can reasonably expect to do in a day. Or a tour of the Capital and visiting a few of the monuments.
Soooo
then you need to ask what you want to do.
Like Art, well then you can choose from the Hirshhorn, the Gugenheim, the Cocheran, the National Gallery (both wings) and a dozen smaller galleries.
Like Pandas we got a few at the National zoo... including our new Panda Cub...
and then theres the Smithsonian... and museums
Air and Space, Natural History, National History, American Indian, Postal Mueseum, Holocaust Museum, and there is a Spy museum that does cost a little cash (but its cool) Im sure I forgot some.
Air and Space has some rocking movies too, IMAX and a sweet planetarium.
The FBI building does a tour
the National Cathedral is beautiful
oooh and the Arboritium is a neat surprise...
There are tours of the Capital, and Mt. Vernon (washingtons plantation on the Potomac is nearby) You can see Arlington National Cemetary and the changing of the Guard at the tomb of the unknown.