rental car and activities
recently returned from 1 week in Cancun.
booked a rental car on Hertz.com for pickup at CUN at a weekly rate of USD $55 using an AAA discount. by the time all was said and done, ended up paying about USD $220 including the local insurance (~ $25/day). other than the price difference, no real surprises. not once did we see any "tourist traffic police" lurking to pounce on unsuspecting visitors for a quick bribe, and we drove quite a bit.
personally, I thought having a car was quite helpful. we stayed in Zona Hoteleria (JW Marriott), but made frequent day trips out to Playa del Carmen (boat to Cozumel), several cenotes (sink holes, and I strongly recommend Cenote Azul - you won't be disappointed!), Tolum (which reminded me of a charming old-school beach town like Santa Cruz, CA).
yes, bring your own snorkeling gear! you'll be using it often!