All of the Smithsonian museums are not to be missed, of course. She'll probably most enjoy the American History and the Natural History museums.
Another favorite experience is seeing the actual Declaration of Independence at the National Archives. They also have an interesting film there about preserving the document & all the changes it has been through.
We went last summer and learned the hard way that lines start about 1.5 hours before opening to see the Capital and the Washington Memorial.
Whereever you end up staying, just make sure you are within quick access to the Metro. While I have driven in DC many times, for a pleasure trip, I always take the Metro in.
I would highly recommend getting ticktes for the Trolley Tour
Washington DC Tours | Old Town Trolley Tours® covering Washington DC metro, memorials, Union Station, Library of Congress, Supreme Court, Washington Monument, The White House, National Archives, Museums. This makes it much easier to get your bearings, and is the easiest way to tour the monuments, as they are rather spread out. Still, you cannot possibly see everything in a day, so you need to try to choose ahead of time which stops you will get off at. The FDR monument is particularly large and fascinating.
I'd allow 2 days for the museums, and a day for the monuments, minimum.