Smtihsonian singular, since it's the institution. The eateries are designed for quick dining by museum-goers. This site, http://www.si.edu/groupsales, has info on special arrangements. I noticed tht the Air & Space museum food court encourages groups to have a picnic on the Mall. There are some restaurants within a short walking distance of some museums, including the Holiday Inn on Capitol Hill. Cafeteria-style isn't bad for grops if everyone is responsiblefor their own payment, since you don't have to go through the hassle of dividing a large bill. The museums of the Smithsonian Institution cover a wide area, so plan ahead where you want to be.

Other museums neaby, such as the Newseum and Corcoran Gallery, may have "nicer" eating facilities. Contacted your Congressional representative to see if you can eat in the House or Senate cafeteria?