We took our kids 12 & 14, on a 6,000 mile tour of the US about 7 years ago and we hit several national parks. My daughter's favorites were Carlsbad Caverns NP especially seeing the bats come out of the cave, and Yellowstone NP, due to the thermal features and animals.
My son, who is severely mentally handicapped, also enjoyed the National Parks. He can't talk but I know he enjoyed the trip. He couldn't take his eyes off the Grand Canyon and kept trying to look over the edge (!) He also enjoyed Yellowstone and even now when he sees video of bison at the thermal features he will stop and watch the TV (and this is a kid who does not watch TV or movies.)
My kids did not have video games or smartphones. DD did have a computer so she could watch DVDs, a life-saver when we were crossing the desert for 8 hours. They enjoyed just hanging out with us, swimming, hiking, and even the driving (except for the desert~)
