Some suggestions:
While we don't go into DC too often because of the traffic and high parking fees ( we tend to eat in Alexandria and Arlington), here are some of the places we enjoy in the city.
Any of Jose Andres' restaurants are excellent. We really like Jaleo for their tapas. Aqua Al 2 is fun. You can get a tasting of five different pastas. Central Michel Richard is another one we enjoy. La Chaumiere is one we often go to for an anniversary dinner. For an informal pizza place, Pizzeria Paradiso is great. Belga Cafe is a Belgian place with excellent food and the "French" fries are really special served with different sauces.