Grand Canyon
Credit: Michele Falzone / Getty Images

The Grand Canyon exposes two-billion-year-old sediments, but remains the National Park's posterchild more for because its photogenic than because its a playground for geologists. Tourists tend to peer into the canyon from the rim, but the smart ones make the hike to the bottom and back. The even smarter ones have mules carry their stuff. 

DON'T MISS: As millions of tourists clog the main entrance to the park along the South Rim gateway, drive over to the North Rim to Tuweep, where adrenaline junkies head to go whitewater rafting and bypass the crowds. Camp for free next to a 3,000-foot drop and hike the park’s shortest, but steepest, trail to the bottom where Lava Falls, the roughest rapids in the whole canyon, await.