I LOVED Yellowstone, but would suggest on making reservations way in advance for an in park lodge or else you will find yourself forever riding in a car on a road hoping for some animals to wander by. We did seee about everything, but bears, the natural wonders are specatacular, but we drove from place to place, and took one bus ride.
I absolutely loved Santa Fe, New Mexico, bad for us with lupus, but if you go in cooler months and wear all the protective stuff you can enjoy the adobe buildings, the cool museums, the oldest house and oldest church in the US. Also the Governor's Palace wear you can buy handmade items from local Native tribes. The scenery is also gorgeous.
I liked Key, West a lot, a different atmosphere, a unique view of the way the water is and lots of interesting things to do in all the keys.
The other places I liked, the Grand Tetons, the Badlands, Pike's peak, The Smoky mountains but only off season and if you can get about in the woods etc.
Of course I must mention Alaska, I have not been there but my cousin lives there while her husband is in the military stationed there and she adores it, my aunt and uncle went and they had wonderful things to say about the scenery, people, Denali National park is supposed to be so beuatiful, they have trains too that take you through the scenery...
Now I'm getting itchy feet and ready to travel....