I'm a DC native and jexjex and Matt Wald are right on target with their recommendations. DBK: the cab ride from the Marriott to 1789 (best rack of lamb in town) should only be about 10-15 minutes, unless you are going Friday evening at 6 p.m. when 30 minutes might be more accurate.
My only addition: Bistrot Lepic on Wisconsin Ave. (upper-Georgetown, lower Cleveland Park, call it what you like)... small, great French, they don't advertise because word of mouth keeps the tables full.