If you're going to visit one place on your Washington DC tour, we suggest the National Mall. Located right in the middle of the city, the National Mall houses some of the most interesting monuments, buildings, and museums in Washington.
While you might not be able to walk everywhere in Washington on foot, you probably won't need a car, especially in the center where most Washington DC tours take place. You can quickly get in and out of downtown Washington without a car, and you'll be able to walk once you make it there.