Dry Tortugas, Florida
Credit: Greig Cranna / Getty Images

Sixty-eight miles across the Gulf of Mexico from Key West, this seven-island archipelago and national park is accessible only by boat or seaplane. The daily Yankee Freedom ferry (departs Key West at 8 am, returns at 3 pm; $170 round trip, including $5 park fee) makes day trips a breeze, though the better move may be to camp overnight on the second-largest island, Garden Key ($3/night). It's entirely pack-in, pack-out – but those willing to do a little work will be rewarded with a nearly empty beach once the afternoon boat ferries away less hardy visitors. "The fact that it takes some effort adds to the allure," says Don Finefrock, executive director of the South Florida National Parks Trust. "You're isolated amid a really pristine environment." Explore Fort Jefferson, a hulking, six-sided Civil War–era barracks and prison, snorkel the islands' protected waters and coral reefs, dive a maze of caves and shipwrecks, or just chill among the preserve's hundreds of loggerhead turtles and rare seabirds.