Great place to watch planes land...

[kamilyun]

I was visiting my brother in Alexandria, VA this week.  My 2 year old loves to point out every "Big Airplane!" she sees and Reagan National is just a few miles away.  We went to Gravelly Point Park (map on page linked below) which puts you about 1500 feet from the end of the runway. What a blast.  Highly recommended if you are ever in the area.  There's also a Metro stop, so public transportation can get you there.

Lots of activity in the area.  Saw Marine One flying up the river, too.

Linky

[OSU]

When I used to live in Maryland (4 years ago), Marine One would always fly right over my house to and from Camp David. On 9/11, my dad heard them and he ran outside to see them. They were so low, they could have blown the shingles off of our roof. The F-16s escorting the choppers were only 200-300 hundred feet above him.

[texasmom]

Super spot 
Lived there for about 6 years... was a favorite spot of ours 
The parkway down toward Mt. Vernon is pretty awesome as well.