I LOVED Yellowstone when we went there as kids. 'Old Faithful' was so cool, and I specifically remember all of the colors in those thermals and Dad telling me there was microscopic life living in that kind of heat!
It's going to be pretty spectacular when she blows! ;)
Another Fun Family Vacation was to Crater Lake National Park in Oregon:
Crater Lake: formerly Mount Mazama before it blew its stack.