Living in Baltimore, I like to take the train to DC once a month or so to try out some of the great restaurants. Here are some of my favorites:
1. Zaytinya - Greek, Lebanese, Turkish Mezze - or small plates. If you go with a small group, you can sample a bunch of different dishes. The wine list is almost all Greek - which I was not familiar with, but the waiter was a great help in selecting something good. www.zaytina.com
2. Ten Pehn - Asian fusion cuisine, but a bit more upscale. Located at 10th and Pennsylvania (get it?) this place is always good. Great Martinis, too!
3. Marcel's - Belgian / French. A bit more expensive, but always good. If you are going to a Kennedy Center performance, this restaurant also has a shuttle to take you there an back. Check out their site to confirm times
4. Blue Duck Tavern - in the Grand Hyatt hotel. EXCELLENT. They call it simple American cooking, but it is not anything I can make at home. This is always a nice splurge.