My experience here was about a month of travelling independently. In terms of national parks and nature, I explored two extremes of the country, California in the west, and Florida in the east. I travelled this area before digital cameras so I don't have much in the way of images.
The U.S.A. is a huge country and therefore has an amazing variety of landscapes, ecosystems and species. The national park system is excellent, as is the interpretation through rangers and books.
The state of California is long and varied. It includes coastal desert, high country desert, rainforest, conifer forest, and beaches. On the coast between Los Angeles and San Francisco, you can visit areas where there are huge Northern Elephant Seals lounging about. Also on this coast is the Big Sur National Park, with huge conifers and species such as the Acorn Woodpecker. In the extreme east of the state is Death Valley, which was starkly pretty, but I didn't see a huge range of species.
In the nearby state of Arizona is the spectacular Grand Canyon, which is so big it doesn't seem real, and when I was there in the winter, had snow on the edge, and mule deer.
On the other side of the country is Florida. Here at the tip of the country is the area called Flamingo, where I spotlighted families of Racoon, and by day saw massive snapping turtles, box tortoises, and manatees.