That's very kind of you to let her choose. Given that it's just the two of you, if money is no (or very minimal) object, I'd recommend Marcel's. It's not hip in the way most of the others mentioned in this thread are, but it's clearly one of the best restaurants in DC. Similarly, if you don't mind a quick jaunt to Alexandria, I'd recommend Restaurant Eve.
What I meant by "money": A five-course tasting menu at Eve is $85; at Marcel's, $110. Seven-course tasting menus are also available: $150 at Marcel's, not sure about Eve. Wine is additional.
Thanks, Alex. I'll add Marcel's to the list. She's actually eating at Restaurant Eve the night before I get up there! We had lunch there during her last visit and fell in love with it!