Dyke Marsh is one of the largest remaining freshwater tidal wetlands in the Washington, DC area, protected along the Potomac River south of Alexandria. A short trail and boardwalk-like sections give access to marsh views, bird habitat, and a quieter side of the river corridor.
It is a good low-key nature stop if you have a car, bike, or time to explore beyond Old Town Alexandria. Bring binoculars if you enjoy birdwatching, especially in migration season.