Here's the Fairfax County Park Authority overview of some of the major parks.
I used to live behind Nottowood park
off Courthouse Rd. (66E to 123 turn right onto Courthouse). It's got some good stuff.
Also check out the green sections of this Google map
of the Fair Oaks area. Those are either parks or golf courses.
Just be sure to make sure there's not too many sex offenders
living around the parks, right?