If your walking shoes are a year and a half old and the cushioning is stomped flat, get new ones before you go. You’ll be doing a lot of walking on concrete and terrazzo. My feet hurt after the first day.
Allow a lot of time to get places on the Metro. We rented a house in upper northwest, so our route to The Mall was to drive about a mile, park three blocks from the Friendship Heights Metro station, walk there slowly (with boy and dog), change trains downtown, go another stop or two to some part of The Mall, then walk to a museum. This took almost exactly an hour.
A Metro map is essential, a Metro iPhone app is optional. I used find a metro dc, which was mostly useful for the list of outages. Unfortunately, they just say “elevator outage”, without mentioning whether it is the train/mezzanine or mezzanine/street elevator. You really do want a simple, free iPhone Metro map.
The best street map of the downtown area is the National Park Service map of the National Mall (pdf). This shows all the monuments, a generous amount of surrounding city, and the exact location of the Metro entrances. You’ll want a paper copy, so drop by the White House Visitor Center and grab one. I expect other NPS visitor centers have them. I got ours at the ranger station just southwest of the Washington Monument.
At any Smithsonian museum, you’ll find a booklet that describes all of them. Donate $2 and get the booklet, because it has the floor plans and exhibits for all the museums. The Air and Space Museum has a bigger, more detailed floor plan at the info desk, so ask for one of those.
Mitsitam Café at the National Museum of the American Indian is the best meal on the mall. It costs about the same as the other museums, but it twice as good and four times as interesting. Though you can get a burger or chicken strips, you can also get bison, pheasant, planked salmon, and other native foods. I had seaweed and wild mushrooms, some great fry bread, and grilled game. It is even a pretty place, with a river-like fountain outside the window.
We checked several guidebooks out of the the library and the house we stayed in had even more, but we ended up almost always using Pauline Frommer’s Washington D.C..