Lake Fairfax, but its more if you have children. But get there when they open either at 9 or 10am. Once full it may take a while to get in. You can always go to Bush Gradens, Water Park USA if its still called that. I think there is one called Grey Wolf Lodge, But I am sure if you googled water parks in Northern Virgina, or just Virginia water parks, you might find a list of them.
Forum:
Fairfax County General