Cancún is easier to get around without a car than a lot of visitors expect, especially if you're staying mostly in the Hotel Zone or downtown.
The R-1 / R-2 city bus
Cheap, frequent, and running the length of the Hotel Zone boulevard between downtown and Punta Nizuc. It's the single best way to hop between beach clubs, malls and restaurants without a car — pay in cash on board, exact change appreciated.
Colectivos
Shared vans that run set routes, mostly useful downtown and for reaching nearby towns like Puerto Morelos. Flag them down like a bus; the destination is usually shown on the windshield.
Taxis
Plentiful outside hotels and on the street. Rates in the Hotel Zone are noticeably higher than downtown, and meters aren't standard — agree the fare before you get in. Hotel-arranged taxis are convenient but priced accordingly.
Renting a car
Worth it if you're planning to explore Puerto Morelos, cenotes inland, or Chichen Itza independently. Roads to major sites are in decent condition; tolls (cuotas) on the Cancún–Mérida highway are worth paying for the time saved over the free (libre) road.
Day trips by ADO bus
ADO's long-distance coaches connect Cancún with Playa del Carmen, Tulum, Valladolid and Mérida on a frequent, comfortable schedule — a genuinely good option if you'd rather not drive.