inexpensive and fun restaurant in Northwest DC - near 15th and F
What counts as inexpensive? Bistro d'Oc is good, on 10th, across from Ford's Theatre. Good solid French food, and they have a pre theatre special.
Perhaps a bit more expensive, but you can sit outside, is Cafe du Parc next to the Willard. If you order drinks and one dish, shouldn't be that bad. Outside area is lovely.
Depends on the meaning of "expensive." I am guessing you are looking for something within walking distance of a particular hotel. Let us know if you are planning to expand your search.
You may want to try Finemondo for Italian on F between 12th and 13th.
Some people think it's overrated, but Old Ebbitt Grill on 15th really is a Washington institution. They have a diverse and reasonable menu, and the atmosphere is normally lively.