Gravelly Point is located right along the GW Parkway just to the north of Reagan National Airport.
It is a perfect place to go and watch the planes land or take off but I advise you to check the weather ahead of time. Now why would I say that?
The wind direction determines which direction planes will take off and land. This is because it is easier to generate lift when heading into the wind. Here's a great website from our friends at NASA to talk a little more about that.
Ok, so what's my real reason for wanting to know the wind direction? Well, on days when the winds are southerly (or out of the south) planes are landing directly over your head and taking off to the south. On days like this, the planes are lower to the ground as they come in on approach, which makes it much louder and a much better experience. Planes climb very quickly when taking off to the north because of D.C. and eventually the buildings they have to clear in Rosslyn. Here are a few examples.
Landing from the north:
Taking off to the north:
You be the judge. Enjoy!