Will be flying into Cancun and driving to Tulum. Any recommendations on rental cars appreciated.

do a search

Avis, Hertz, Budget, Thrifty & Dollar - all have rentals at/near Cancun Airport.

If you want to save a bundle, go with one of the local car rental agencies like Easyway or America. They can rent you a reasonably nice car with all insurances (which you definitely want) far cheaper than the big guys. The only drawback is that they pick you up at the airport and shuttle you to their office a few miles away.

I always rent from Easyway and take all the insurance, their office is only about a 5 minute drive away off the 307. For an extra $5/day you can get insurance to cover the tires and glass, which is not covered under the regular insurance.

Book directly with them, the horror stories I read from time to time usually involved people who booked using Expedia, Hotwire, etc.

We rented our car thru Autoeurope and had no issues at all. It's easy as all the insurances are included and you prepay so we don't have to worry about additionals when we pick up the car. The car was from Europcar and was easy to dropoff at Cancun airport although it is not signposted. The dropoff time was about 4am, they dropped us off at the terminal after we handed in the car.
You can have a horror story no matter where you book.