If you are disabled or a senior citizen, you qualify for a free lifetime pass into any of the National Parks. And the free pass does not only cover you, it covers all the people entering the parks with you, as long as the pass holder is in the car - you don't even have to be the driver.
But the lifetime free pass goes even further - it entitles the pass holder to stay at any National Park campground for half price. You apply for your lifetime pass by visiting any of the National Park headquarters. They will give you a short form to fill out and ask to see two pieces of paperwork showing you are either a senior citizen or disabled, that's it, they give you your free pass right than and there.
These free passes have been available for a very long time but the Park Service keeps it low on their radar and most people don't even know about them.