Cancún

Cancún needs 4 days: the Hotel Zone with its white sand beaches for most of it, and a Chichén Itzá or cenote excursion for the rest. December-April is the dry season with lower humidity, avoiding hurricane season (June-November). All-inclusive resorts are pricey by Mexican standards, though still cheaper than the English-speaking Caribbean. It's the gateway to the Riviera Maya, and its sand, made of ground coral rather than silica, stays cool to the touch even under the intense Caribbean sun.