A 26-mile-long spit of sand 40 miles northwest of Cancún, Isla Holbox has evaded the vagaries of mass tourism: There are no roads here, only sand. No cars, only golf carts. And no day-trippers—just 1,500 residents and a few smart vacationers. Take a shuttle from Cancún to the mainland gateway of Chiquilá ($175 each way; holboxisland.com). From there it's a 15-minute ferry to Holbox ($4 each way; holboxmonkeys.com.mx). Your base is Casa Sandra, a resort with a series of ocean-facing villas (doubles from $155; casasandra.com). May through September is whale shark season in the Yum Balam Biosphere, a 600-square-mile protectorate of coastal waters and forests. Explore the menagerie with a snorkeling guide from Mextreme Travel—the outfitter donates a portion of proceeds to whale shark conservation ($50 per trip; mextreme-travel.com). Tip: Bring cash. Holbox has no ATMs, and credit cards are rarely accepted.