Take a guided tour to orient yourself to Washington DC, and to see various photo opporunities - Old Town Trolley Tours  has received positive recommendations from visitors, although OTT might get the edge because they seem to have more experienced full-time drivers and guides, while the Tourmobile appears to use part-time student help in the summer months.  You'll drive by the Capitol, the White House, the Supreme Court, and all the major monuments.

In addition, both tours have hop on-hop off privileges which allow you time to spend several hours at a museum of your choice - the Smithsonian Institution operates sixteen of them at last count, with the majority of them located on the Mall.  If you're not museumed-out at the end of the day, the National Archives has evening hours in the spring and summer months.  Note that the tours end at dusk, but you can take the Metro back to most hotels in the City.  Also, both the Old Town Trolley and the Tourmobile offer evening tours in the summer.

For dinner options, try the Rooftop Terrace cafe atop the Hotel W, which offers great vistas of the  White House lawn and the Potomac River.  Other options include the numerous restaurtants in Chinatown and the Penn Quarter, between the National Archives and the Gallery Place Metro stations. 

Off-Site Links:

http://www.archives.gov/national-arch...
http://www.historictours.com/washington
http://www.trustedtours.com/store/Was...
http://www.si.edu/museums