Old Town Alexandria is very walkable — most of the historic district can be covered on foot in an afternoon. The King Street Trolley runs a free shuttle between the Metro station and the waterfront on King Street, which is useful for the first or last stretch.
The Potomac Riverboat Company operates water taxis from the Old Town waterfront to Georgetown, the National Mall, and National Harbor — a practical and enjoyable alternative to driving or Metro for reaching Washington. Boats run in season from King Street Pier. For Mount Vernon, the Mount Vernon Trail follows the Potomac south on a dedicated bike and pedestrian path the entire 15 miles from Alexandria to the estate.