My friends and I are spending a week in Cancun this summer. Is renting a car necessary?

We'll be spending time on the beach and relaxing but we also would like to get out of Cancun and explore more into nature (ie. see animals, caves, jungle, etc.).Do the shuttle systems in Cancun venture far outside Cancun? Could we rely on busses and shuttles or is renting a car necessary for doing everything we'd like to do? Also, how much do car rentals generally end up costing?