On a weekend? In the fall, outside of the main tourist season? Drive.
Do note that this Sunday is the Army Ten-Miler race which will create a big old mess in its immediate vicinity before, during, and after. So avoid the race course with a car.
Note that on Saturdays the meters run in DC; however they are free on Sunday. You should be able to find a decently cheap lot to park in on Saturday and street parking on Sunday. What parts of the city are you planning to see, and how much walking do you like to do?