I'll second the Marriott. Good location with a few good restaurants nearby, plus an easy walk to all the bars in Adams Morgan.

Also, the National Zoo (free) is a 5-minute walk from the hotel.

Hilton's got a few hotels near Dupont Circle (one stop further than the Marriott) including the Hilton Washington and the Hilton Embassy Row. Both are nice if you're a Hilton fan. Dupont Circle is also a great area for bars and restaurants.