Hotel Harrington is probably a good choice. I have not stayed there but my cousin stayed there and thought it a good value. It is convenient.
Capital Grille is good for business dinners, but I think it is a chain now (at least I know they have them in Atlanta and other major cities). My favorite downtown spot is 1789, in Georgetown. It is on the pricey side, but they have special priced dinners during the week, and they also have an early "theatre special" on Saturday that is quite resonable. Last time our admin assistant ate there Alan Greenspan was at the table next to her. It is special occasion, but not stuffy.
Dupont Circle has lots to choose from as well.
http://www.1789restaurant.com/main/index.shtml
Near the Harrington there is The Shakespeare Theatre. If a show is playing I highly recommend (we subscribe)and I recommnend "Teaism" for a quick lunch nearby.
National Cathedral tour is one of my favorites and drive by all the embassies (free)
Museum of the American Indian (Smithsonian) and The Spy Museum are new and have longish lines.
Rent a bicycle and ride the bike paths along the Potomac or the C&O canal very scenic and no cars!
Not sure if you are an NPR listener but their studio tour is also interesting (near Chinatown).
Will try to think of more (figure you have the basic Smithsonian, FDR, Wash Monument, Viet Nam, etc. covered)