Several notes on renting.....take the extended insurance as your insurance will not cover Mexico the way they demand it. If something happens they require all costs to be paid up front before you leave the country. The extended insurance is expensive but worth the peace of mind. None of that file for reimbursement stuff in Mexico.
Also, book online before you go. The prices are higher when on the ground in Cancun than online 30 days ahead.
I'd suggest a minivan, too. You will not find many SUV's for rent, frankly. Most people drive small cars and you won't find anywhere to park the danged things anyway. If you are thinking of driving around Cancun, reconsider. Parking is a nightmare and drivers (taxis and buses) drive like New York City or Rome. People take the buses and cabs all the time instead. For road trips driving is not a problem because you can get immediately out of the city and then things settle down. We always rent a car for a few days during our trips for sightseeing flexibility. We had 5 adults this trip and just made do with a Nissan sedan. It was tight but lots of fun and got decent gas mileage (gas stations are also few and far between if going off teh beaten path).
Cheryl