We visited three National Parks; Grand Canyon, Yosemite and Muir Woods, during our 2019 Trip. Each of these has an admission fee, but you can save on that by purchasing an America the Beautiful annual pass.
The pass costs US$80 plus shipping from this site: https://store.usgs.gov/pass
Shipping to Europe was US$20. It may be that you could purchase one of these at the first point of entry – but I wanted the pass in my hand. And it’s an actual piece of plastic, like a credit card and also comes with a thing you can use to display the pass in the vehicle windshield.
The pass provides free entry for the vehicle and passengers to a huge range of US National Parks. Things like parking fees, which you have to purchase for Muir Woods, aren’t included, but the pass still saves plenty, even for the three parks we visited.