Great Falls Park is closed at dark. Overnight camping is prohibited and there are no lodging facilities within the park.
There are many options for lodging within the immediate area. A web search will provide a variety of choices. Nearby towns include Reston, Herndon, Tysons Corner, Falls Church, Arlington, Alexandria, Sterling, and Washington D.C.
Nearby Camping:
Fairfax County Park Authority
Northern Virginia Regional Park Authority
Chesapeake and Ohio Canal NHP
Prince William Forest National Park
Shenandoah National Park
Catoctin Mountain National Park
Greenbelt National Park