We usually rent a car while in Cancun. Taking pictures is a great idea because they will try to charge you several hundred dollars for existing damage. I always take the extra insurance! If someone steals the car and drives it off a cliff I want to be able to walk away. It does cost more but that's OK with me. We stayed at Fiesta Americana which is all inclusive and overall a beautiful resort, but also like to drive to other places just to look around. We were there for 4 days and I think the car cost around $250 US. When you consider the extremely high cost of a cab it is much cheaper to rent. We met some friends at Puerto Adventuras for dinner one night (about 60 miles south of Cancun) and chose to take a cab. I knew I would be drinking (a lot) and didn't want to drive. The cab fare was $100 each way. A lot of dough, but more peace of mind.
Anyway, driving in Mexico is not a bad experience, just lots of traffic. Have fun!!