Polihale State Park

"Polihale Spectacular sands—making up the longest stretch of beach in Hawaii—sweep for 15 miles starting here, fringed by postcard-perfect turquoise water. This patch of ocean is rough, rowdy, and perilous: Watch for sneaker waves as you stroll along the coast. Swimmers should stick to the Queen's Pond, an area girdled by a protecting reef. But the big attraction at Polihale Beach remains the dunes, which can pile up to 100 feet high. In summer, hot sand can slip into your shoes, causing burns; savvy beachgoers wear only wool socks, according to Andrew Doughty, author of the excellent Ultimate Kauai Guidebook. Make sure to pack plenty of water—and also an umbrella, if you plan to linger."