I suggest Sette Osteria. It is a wonderful Italian restaurant/brick oven pizzeria that is open to midnight on Saturday. However, I think that they only serve pizza from 11PM-Midnight.
http://www.setteosteria.com/
Kramers Bookstore/Cafe in Dupont is open 24 hours on weekends.
Old Ebbitt Grill on 15th Street is also open pretty late if you feel like cabbing it there.