Free entrance to National Parks this weekend. Details here.
Participating parks in Virginia
Appomattox Court House National Historical Park
Arlington House, The Robert E. Lee Memorial National Memorial
Assateague Island National Seashore
Colonial National Historical Park
George Washington Memorial Parkway's Great Falls Park
Manassas National Battlefield Park
Petersburg National Battlefield
Prince William Forest Park
Shenandoah National Park
Go here for a list of participating parks by state.