Thanks for all of the comments guys, I appreciate them
Maybe I'll stay away from the rentals and take Aaron's advice and just rent a jeep from across the street if it comes down to it.
We're the kind of people that like to roam around rather than being stuck, but we've never been down to Cancun, so this time around we'll stay away from the rentals. I got a $508 quote for a week rental of a mid-sized car. I suppose that that could go a long way in cab fare