Baseball, soccer, basketball, lacrosse and more .... Centreville has a world class youth sports organization - Southwestern Youth Association (SYA) 

And so does Chantilly - Chantilly Youth Association (CYA)

Lots of opportunities for adult to play too. Click here for adult leagues in Fairfax County. (Other children's leagues also listed.)

Local golf courses: Fairfax National(Semi-Private/Public); Chantilly Country Club (Private) ; Twin Lakes(Fairfax County) 

Cub Run Recreation Center has a pool, gym, and lots of fitness classes (and nature programs too).

E.C. Lawrence Park has trails and connects to the Rocky Run Stream Trail (great nature programs too).

Bull Run Regional Park offers: Camping; Trails; Trap & Skeet and Archery Range and more.

And there's a wealth of opportunities to watch professional level competition:
   Washington Redskins (NFL Football)
   Washington Nationals (NL Baseball)
   Washington Capitals (NHL Hockey)
   Washington Wizards (NBA Basketball)
   Washington Mystics (WNBA Basketball)
   D.C. United (MLS Soccer)
   Woodbridge, VA Potomac Nationals (Minor League Baseball)