Of all the beaches on the Big Island, the unspoiled crescent of black sand at the base of Waipio Valley is easily the most secluded: it’s sheltered by 2,000-foot cliffs and it’s a grueling three-mile hike down. But don’t be dissuaded: you’ll be rewarded handsomely for your efforts with superfine, inky sand and calm, clear turquoise water. Look to the east, and you’ll spot the spectacular Kaluahine Waterfall (pictured) and the larger Waiulili Falls, which can be accessed by a rocky trail…