Colorado's Great Sand Dunes National Park is like a giant sandbox for adults, featuring the highest sand dunes in North America at the tallest dune towers 750 feet high, as well as stunning views of the Sangre de Cristo Mountains.
Glacier National Park is a winter warrior's paradise, with snow that lasts well into early summer. The popular, 50-mile Going-to-the-Sun Road boasts harrowing mountainside driving and unmatched views of the park's glaciers and carved valleys.
Zion National Park in southwestern Utah is known for its breathtaking red sandstone canyons. The Narrows in Zion Canyon is a popular hike, featuring walls that are 1,000 feet tall and a river less than 30 feet wide.
An easily accessible park from much of the East Coast, Shenandoah National Park is a wonderland of vistas and calming waterfalls in Northern Virginia. The park is in the Blue Ridge Mountains, and a portion of the Appalachian Trail runs through it.
At Kenai Fjords National Park in Alaska, visitors can get up close and personal to marine mammals. If they're lucky, they'll also catch a glimpse of one of the park's 40 glaciers "calving," meaning that massive chunks of ice will break off the main glacier and crash into the bay below.
Saguaro National Park, home to the almost comically large cacti of the same name, is a bastion of the American Southwest. In the Sonoran Desert surrounding Tucson, Arizona, it's home to desert wildlife as well as historic petroglyphs.