Are you planning to drive throughout the city and then drop it off at an Avis location in DC? Unless you are planning to go to places that the metro is lacking or tough to get to by public transportation( Annapolis, Mount Vernon(well aware of the buses from Huntington Station), WV, Everything south of Springfield, Frederick etc I don't think having a rental car is worth it. Hotels in DC charge a lot for nightly parking. Then you have awful city traffic and then so-so parking availability at best on the street and parking lots. Then most of the sites anybody would ever see is close by plus metro accessible.