National Museum of Women in the Arts may be a possibility. They have a cafe for lunch + Sunday brunch. Website says they do "private lunches." Very cool building. Near Metro Center.

To be honest, pretty much all the food I've had in the Smithsonian or in House/Senate office buildings has been unremarkable...

The Newseum has a high-end restaurant The Source, which is quite good, though pricey. But it isn't anything symbolic--just a restaurant.

There is the indoor courtyard in the National Museum of American Art/National Portrait Gallery -- the food is overpriced museum fare -- sandwiches, etc. cafeteria style.