Ellanor C. Lawrence Park covers 650 acres of second-growth forest and stream valley in the southern part of Chantilly. Fairfax County operates a nature center on the grounds that runs programs on local ecology and wildlife. The trail network is modest — a few miles of well-marked paths through mixed hardwood forest — and the terrain is gentle enough for children and casual walkers. Two historic farmsteads, the Walney and Cabell's Mill sites, are preserved on the property, though the structures are in varying states of repair.
This is a local park used mostly by nearby residents, and it functions well as one. It's not a destination worth a dedicated trip from Washington, but if you're spending time at the Udvar-Hazy Center and want somewhere quiet afterward, the park is a reasonable place to walk for an hour. The creek corridors hold good bird habitat in spring migration, which draws birders from across the region.