Great Falls Park is where the Potomac River cascades over jagged rocks through Mather Gorge.
The main visitor center for the park is on the Virginia side of the river and is where most of the infrastructure and hiking trails are located. On the Maryland side of the river is the Chesapeake and Ohio Canal National Historic Park and the Great Falls Tavern Visitor Center. There are a dozen unique passport to the National Park cancellation stamps so be sure to check the park web site for details. There are a lot of hiking and walking opportunities here as well, the most fun is the Billy Goat trail where you literally go up a cliff face like a mountain goat.
For the less adventurous there are plenty of nice easy strolls that make for a very pleasant summer afternoon.
Be sure to check out the locks and restored buildings.
Just a few miles from the Washington DC metro area is this wonderful series of parks offering both history and a relaxing rural setting.