I'll second this. It has been a while, but after going to a wedding in PA, a quick stop at Hershey Park, we drove down to finish the trip in DC. After missing a street, circling around to get to the hotel, and probably upsetting (putting it mildly) at least a dozen drivers in the process (DC drivers aren't the least bit hesitant to use their horns), after checking into the hotel the first thing we looked into was an early return of the rental car. We had picked it up at BWI, but they let us return it to DCA early, no extra fee, and even didn't charge for the days we hadn't used.

The metro in DC is wonderful to use, we used it on that trip above (while staying at a hotel off of DuPont circle), as well as another trip staying out in the Maryland 'burbs (hotel was across from a metro stop). What took a friend a $50+ and 2-hour cab ride (in rush hour) took me a $2 and 45-min metro ride.

And getting to/fr the metro at DCA is such a breeze ^