On a recent hike in Shenandoah National Park, my wife and I enjoyed cooling off in a nice swimming hole. Swimming holes are excellent lowtech alternatives to modern pools with electric pumps, chemicals, and expensive membership fees.
Swimmingholes.org is a good resource to find a swimming hole in the United States. I know of at least one good swimming hole in Northern Virginia that is not on the list, but sometimes things are just better kept secret.
See Also: Lowtech Substitute for Air Conditioning