The hotels and resorts can arrange transportation for you to the sites and attractions in Cancun. Let them take care of that if you’ll spend the majority of your time on the beach or in the resort. If your goal is to explore, rent a car and get directions for every site you intend to visit. Taxi rates are set by how many "zones" you travel through to get to your destination. There are no meters in the cabs. This way you can get a price up-front from the driver as to how much the fare will be. Rates are posted in most hotels near the front desk.
NOTE: Taxi rates are more expensive from the front door of your hotel than if you walk to the street and flag down a taxi [hotels get a kick-back]. The cheapest way to get around is by public bus. The public bus system operates 24 hours a day, 7 days a week every day of the year.