Yellowstone was the world's first national park, and 150 years later it's still the most concentrated wildlife-and-geothermal show on Earth. Half of the planet's geysers erupt inside its 2.2 million acres, along with rainbow-hued hot springs, roaming bison herds, gray wolves, grizzly bears and one of the last major elk migrations left in the Lower 48.

The park works like a giant figure-eight road called the Grand Loop, tying together five distinct regions. Old Faithful and the Upper Geyser Basin are the most famous, but the Grand Prismatic Spring, Mammoth Hot Springs terraces, and the Grand Canyon of the Yellowstone (with its 300-foot waterfall) are equally unforgettable. Lamar Valley in the northeast is the best place in the world to see wolves in the wild.

Yellowstone is a driving trip, so plan a full week if you can. Base yourself inside the park's historic lodges (Old Faithful Inn is one of the great national-park hotels) or just outside in West Yellowstone, Gardiner or Cody. Combine it with Grand Teton National Park an hour south for the ultimate Rocky Mountain road trip.