The metro in DC is fantastic (I highly recommend it), you won't need a car. Also- the cabs don't have meters, they work in zones and charge a flat fee no matter where you are going.
-Stay at the Sofitel by the White House:
http://www.sofitel.com/sofitel/fiche...he_hotel.shtml
-Go here if you like beer (most in the world):
http://www.lovethebeer.com/brickskeller.html
-Check out the nightlife in Adams Morgan (if it's nice out, sit on the roof deck @ Reef)
-Check out the nightlife in Dupont Circle (Frontpage is fun)
-Walk around Georgetown (also check out the nightlife- I recommend Old Glory)
-Eat at La Ferme if you don't mind expensive (it's actually in Chevy Chase, MD- you will have to take a cab but it's totally worth it)
-Eat at Mi N Yu in Georgetown (you will also need to take a cab here- the metro doesn't go to Georgetown)
-Eat at 5 guys if you like good/cheap burgers
-Go do all the tourist stuff (the museums are all free)
Give me some time and i'll think of more.