Gravelly Point

After wandering on the mall and smelling the Cherry Blossoms, a quick trip to National Airport to watch the planes land at Gravelly Point (wikipedia), a park 1400 feet from the end of runway 19 at National Airport (Google Maps satellite image). You could drive there (hint: northbound on the George Washington Parkway), or you can metro -- go to National, walk through the terminal to the northern most doors, go outside, walk by the maintenance buildings, cut through the parking lot after the last building and pick up a bike trail the goes to the park.