You’re right about Yellowstone. I’ve been to scores of national parks and Yellowstone, hands down, is the best of them all. There were at least ten attractions that could be parks all by themselves. I loved the bubbling mud and the sulfur cayldron.
My second favorite is fairly near there, Craters of the Moon. It still doesn’t compare to Yellowstone though. The people are great out there too.
I did a car trip from Chicago to San Diego with a Belaruse immigrant friend who had very little patience re savoring natural wonders, but he just loved all the action and variety of the thermal features behind Old Faithful.