Yellowstone has limited access by road, but you could hike into it.
The Grand Canyon has visitor’s centers and a few established areas with infrastructure for various activities, but you could hike to it, but getting into the canyon is another matter.
The other thing is that going in by road and paying for a pass let’s people know you are there and if you haven’t come back. Both areas are dangerous and people get in over their head because they have no understanding of the dangers of nature.
Playing with fireworks, firearms, or chemicals can be a solid way for anyone of any age to ruin their life.