I've been to Cancun six times. Haven't ever rented a car. So long as your hotel is in the Zona Hotelera, the bus will get you up and down Kukulcan Boulevard 24 hours a day for a paltry 6 pesos each way. That's the way to go. To get to the ruins at Tulum or Chichen Itza, just line up a day excursion with one of the many activity touts. As for restaurants, most of the non-chain, local restaurants will be found downtown, which you can also access via bus.