If you're in the mood for a splurge, try Obelisk, very near Dupont Circle, P St between 20th and 21st. I ate there about once a month until about two years ago (my monthy trips to DC are now one-day turnarounds instead of overnights) and was never disappointed. I strongly urge you to make reservations, as the place is fairly small. The menu is small and changes daily, and they have a very good and fairly priced wine list.

If you don't want to spend about $50 on dinner, the whole P Street-Dupont Circle area is crawling with restaurants. Lots of bookstores and cafes in the area, too.