This is a good infographic that shows who can get what sorts of passes: https://www.nps.gov/planyourvisit/images/Passes-Flowchart-up...

For example, it's also free for current military and veterans, or if you volunteer for 250+ hours that year

Some states also let you check out state parks passes (NOT for the National Parks, but the state ones) at libraries.

Washington: https://discoverpass.wa.gov/148/Check-Out-Washington California: https://www.parks.ca.gov/?page_id=30806 Nevada: https://parks.nv.gov/about/library-pass