18. Channel Islands National Park, California

Annual visitors: 366,250

Fun fact: Signs of human habitation on the islands date back to 35,000 years BCE

Off the California coast, this archipelago of five islands is a chance to step back in time and see the West Coast as it used to be. The ocean surrounding the islands is a prime hangout for sea lions, and whales can be spotted migrating at certain times during the year. Favorite activities for visitors include camping, diving, and kayaking through the sea cave dotted throughout the park.