Disabled veterans can get free access with an Access Pass on their cars, granting free access to anyone in that vehicle. On top of access, the access pass gives holders a discount on expanded amenity fees at many National Parks sites, which can include campsite fees, swimming, boat launches, and group tours. More details can be found at this link.
You cans search the National Parks list here.
Please contact a site directly if you have a question about pass acceptance and fees.
The Forest Service, the National Park Service, Fish and Wildlife Service, Bureau of Land Management, Bureau of Reclamation, and US Army Corp of Engineers honor the Annual Pass at sites where Entrance or Standard Amenity Fee(s) (Day use fees) are charged.