For lunch, if you're touring, you're out of luck during the weekends, but weekdays there are a lot of options. The Bread Line is great for sandwiches. And if you find yourself near the Iwo Jima memorial, you can head to Ray's Hell Burgers (best burgers in DC - and they cook to-order). On weekends, lunch is tough to come by, but the Museum of the American Indian has some good selections (I like to just order a bunch of side dishes - also, some of their soups are pretty darn good). By the Newseum, I *think* The Source is open for lunch on Saturdays, and there's a restaurant near the Spy Museum that's supposed to be OK (and presumably is open on weekends).
For dinner, if you want a memorable meal, see if you can get reservations to Komi (but call NOW!) Best meal -and service- in all of DC (in my opinion). Other nice options (and easier to get reservations for) include Palena or Poste Brasserie (the 20 Bites dinner is fun). Hot tickets (besides Komi) include Minibar at Cafe Atlantico (I've never scored reservations there but it's supposed to be quite memorable) and more recently at Graffiato's (opened by the former Top Chef contestant). And if you are willing to cross a bridge, there are a many nice (and moderately priced) options in Clarendon, VA, including Liberty Tavern and Eventide.
Ray's Hell Burger
The Liberty Tavern
OMG. Went for the 20 Bites a couple weeks ago. The heirloom tomato dish we had was the best dish of the night. Beautiful tomatoes, basil ice cream, tomato granita -- it was spectacular. They also have a special Heirloom Tomato 20 Bites (haven't had it, but am certainly tempted).