Yellowstone National Park
Even if Yellowstone didn't hold the incredible distinction of being the world's first national park, the 2.2 million-acre park could easily stand on its own for its unique natural attractions and diverse geography. What's more, this behemoth national park stretches across three states: Wyoming, Montana and Idaho. Here, travelers can get lost in the splendor of Yellowstone's many lakes, mountains, buffalo-filled valleys and, of course, its unmatched geysers and hot springs. To avoid the crowds that often plague the park, plan to visit in April, May, September or October.