WOW. Yellowstone National Park was absolutely incredible. Old Faithful is the most popular attraction, but the other geothermal areas (springs, basins, etc.) were also quite stunning. The Bison herds were amazing to see as well. If you ever get the chance to explore Yellowstone, be sure to dedicate an entire day to exploring the expansive park (but please, stay on the trails). Most importantly, take it all in - it's gorgeous.