Why go
A little geographic orientation makes the choices clearer. Cancún is north of the Riviera Maya, while Isla Mujeres lies offshore. Playa del Carmen and Tulum belong to the coastal journey south; Chichén Itzá lies inland in the neighboring state of Yucatán. These are distinct excursions rather than nearby city sights.
The full story
Cancún belongs to Quintana Roo, the state along the eastern side of the Yucatán Peninsula. The peninsula separates the Caribbean Sea from the Gulf of Mexico, and Cancún faces the Caribbean near its northeastern corner. This position explains the city’s relationship with both offshore islands and the long coastal corridor to the south. Within Cancún, the Hotel Zone occupies a narrow strip between the sea and Nichupté Lagoon. Downtown, also called El Centro, spreads across the mainland. A beach address and a central-city address can therefore mean quite different daily journeys, even when both are described simply as Cancún. Beyond the city, Isla Mujeres is reached by ferry from Puerto Juárez. Southward, the Riviera Maya connects coastal towns including Playa del Carmen, Akumal and Tulum. Inland archaeological destinations introduce a different landscape and travel pattern. Decide which of these settings matters most before dividing time among them.