By Metro is the best option for Arlington, Alexandria, and Rosslyn. The Blue, Yellow, Orange, and Silver lines serve most visitor destinations. Trains run frequently from early morning to midnight (later on weekends).
By car is necessary for Manassas, Mount Vernon, Great Falls, and any site off the Metro network. Rental cars are available at both airports. Parking is generally easy outside of Old Town Alexandria and the Arlington centres. Traffic on I-66 and I-395 is brutal during rush hour — plan accordingly.
Cycling is excellent along the Potomac. The Mount Vernon Trail runs 26 km from Roosevelt Island to Mount Vernon, passing Old Town Alexandria, Reagan National Airport, and Gravelly Point. Capital Bikeshare docks are at most Metro stations. The trail is well-maintained and almost entirely off-road.
Rideshare (Uber/Lyft) is reliable throughout the region and often faster than driving in heavy traffic for short hops.