1) Food on the national mall is generally lousy (because it's museum food) except for the National Museum of the American Indian (a Smithsonian institution, located 3rd and Jefferson, or the SE corner of the Natl. Mall). The NMAI has an awesome cafeteria, featuring Mexican, South American, and Native American-inspired cuisine. Definitely the best museum food I've had. Long lines move quickly here.... :)
2) For a good view of the city and everything in it, head to the Old Post Office on 11th and Pennsylvania. There's an elevator to the top and a good view of everything. Nothing is taller than this view except for the Washington Monument, which is difficult to get tix for. Moreover, you won't be able to see it if you're inside it! So, I highly recommend getting to the Old Post Office for an excellent 360-degree view!
3) The National Gallery of Art's Sculpture garden (to the south of the Natl. Archives) is an excellent place to take a break and sit in the shade. They have food, a huge pool/fountain, and had a big jazz band playing when we were there at about 6pm on a Saturday. Plus, the sculptures are really good, too!
4) Please do not miss out on the National Portrait Gallery (on F and 8th, across the street from the International Spy Museum). The portrait gallery was easily the best museum I saw, and it has an interesting courtyard and cafe. It's actually two museums (the museum of American Art shares the space) but if you see only a little bit, please head up to the Presidential Portraits wing in the Portrait Gallery. I went twice and was blown away both times... You will not be disappointed.
5) If you only get to see one monument at night, I would recommend the FDR memorial. Take a cab, if you need to (they're pretty cheap in DC) but you won't regret it. It's a chronological walk through his 4 terms in office, so make sure to start at the correct end. Some light reading helped me with interpretation (check http://en.wikipedia.org/wiki/
6) Beware that sometimes around the mall, finding food is difficult. On the weekends, most of the restaurants downtown will be closed. A relatively safe bet is to head via taxi or pedicab (we paid 15 bucks plus a 5-dollar tip to get from the US Capitol to the Portrait Gallery, about 8 blocks away) into Chinatown (6th&7th Streets, between about E and I Sts.). Chinatown has TONS more food than just Asian, and tends to have open restaurants all the time (whereas museum food always closes when the museums close at 6 or 7pm).