Just a few odds and ends not yet covered:
For that quick lunch while on the mall or museums: Head over to the Old Post Office Pavillion rather than eating from street vendors or the museum cafes. Better food and better prices.
For an entertaining evening of drinks and intrigue: Check out the International Spy Museum. Not part of the Smithsonian's and has a door charge, but some of the evening funtions are lots of fun.
For a bit of night-life (if that's your thing): Adam's Morgan area. Lots of restaurants, live music clubs, etc. to choose from. Madams Organ in Adams Morgan offers blues, bluegrass, rock, etc. depending upon the night. Typically, the bluegrass and jazz is on weeknights with blues or rock on the weekends. Can get a bit crazy with the overconsuming college kids on weekends, but a weekday night with bluegrass...
If you like animals, consider the zoo.
Relativily new attractions: Native American History Museum; National History (just renovated); Air and Space addition at Dulles.