Yellowstone is by far, one of the coolest places we've been. Not quite as awe-inspiring as the Grand Canyon, but there's much more to see.
Since Yellowstone is located in an area of high geothermal activity there are lots of hot springs, geysers, and steaming pools. The artists' paint pots are opaque pools brilliantly colored by the different minerals found in each one. Prismatic Springs is a very large and steamy spring with surrounding seeps of cyan and opal. There are vast mats of cyanobacteria spilling color across the area in between. And the unfamiliar landscape is dotted with geysers that bubble, steam, and sputter constantly.
They also have these hysterical signs warning you not to step on the areas of geologic activity. The illustration depicts a child getting burned with a steam blast while his mother looks on in horror and his dad is completely oblivious. Check the pics.