IMHO, you are probably better off taking a taxi or bus down the coast from the airport. Then just rent cars on the days you decide to do some exploring. You can rent at either Akumal or Playa. For example:
http://www.locogringo.com/dynamic/services.cfm?type=Car%20Rental&location=akumal

If you do decide to rent at Cancun, after much research (all Cancun car rental agencies get generally lousy online reviews) I settled on Easy Way. So far so good with them handling our reservation.
http://www.airportcarrentalcancun.com/index.php

We are renting a car at Cancun because we are driving all the way down to Majahual, but I wouldn't have one all the time if we were just staying in Akumal or Playa.

Hopefully you'll get some experts with better knowledge of the area to answer.

i would rent a car just for those times when I need it.

We have done the same trip you are plannning many times. I would rent a car at the airport. We have used Easyway and America. Both are inexpensive and supply all the insurances. They will meet you at the airport and shuttle you to their rental office. Drive to Akumal, which is quite easy. Do your exploring with the car while staying in Akumal. You can easily visit Tulum, Coba, some cenotes,caves, etc. Take the car to Playa, and return it there (most renal companies will allow this). You won't need a car in Playa. To get back to the airport, you can catch the bus in Playa, which is inexpensive and surprisingly comfortable, or take a taxi.

Another vote for Easyway, used them 3 times with no problems at all, also once used Cancun Renta Car (affiliated with America), same good service and car. Last couple of times I took a private transfer with USA Transfer only because a friend I was sharing a condo with got there earlier than me and picked up the car from Easyway, and just paid my share of the costs. If you want to drop the car off in Playa, Easyway also has an office there.