The first place that came to my mind was the Ritz-Carlton in Georgetown. The lounge area with oversized chairs/sofas, soaring ceilings and exposed brick walls, and a fireplace is perfect for a small gathering. It is, however, somewhat sedate (no crazy drunken college kids here) and the atmosphere is elegant. This is also probably more suitable for post-dinner.

For dinner, not knowing how much you want to spend, I've always enjoyed Buca di Beppo in Dupont Circle (a mile east of G'town). The food is average at best but they cater well to large parties (big portions, good service, lots of birthdays here) and the limoncello shots are fierce. It's always a night to remember (or not, as it were) at Buca.

If you're not confined to Georgetown, Helix (2 miles or so east of G'town) has a hipper-than-thou bar complete with white-washed furniture, lovely banquettes with sheer screens, and fabu-tinis (you can even get one named after your daughter!). But the best part is the outdoor patio. Nothing is better than drinking under a starlit sky

If I was turning 21, I'd want to party with my closest friends at a bar or loud restaurant, get thoroughly sh*t-faced, and end up at a place where my dollar bills will come in handy (I have a perfect spot for that too!) But then again, your daughter may not be as trashy as me

Congrats on your daughter turning 21 and for being a cool dad!