Here's the Fairfax County Park Authority overview of some of the major parks.
http://www.co.fairfax.va.us/parks/glance003.htm
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?