Planning a trip to DC. Rent a car or public transportation?

My wife and I just got back from DC. We used the Metro subway system and did a double decker bus tour one day, during the week, to see all of the sights close to the Mall. On the weekend ( Fri., Sat. & Sun.), we rented a car from Enterprise for $20 a day, the weekend rate. We used the car to go see Mount Vernon, Manas Civil War Battle site and to Annapolis. Those were my favorite parts of the trip. So, unless you are planning to get out of town just use the Metro, Taxis and tour buses. Oh, we did take a Pedicab one hot afternoon on the Mall, that was fun.