I took the ferry to Isla Mujeres today. Talk about amazing--next time I come here, I want to stay there.
I've determined that Cancun is for the 20-30 crowd (very lively nights) and the "I want to be pampered" crowd (tremendous resorts). I'm in neither of those groups. I'm in the "where could I bring a pre-teen with assurance that he would have a good time" group, and Isla Mujeres is just the place. It's small--can't get lost, and nothing is too far away--and has great beaches. I never left the North Beach area, and I understand there are a couple of bigger hotels a little further away, but everything I need is right there at north beach.
Golf carts and scooters seem to be the prevalent mode of transportation there. And bicycles. It seems very family-friendly, and is much more like "real" Mexico than is the hotel zone of Cancun.
Maybe I'd stay there a few days, and then at the Riu Caribe for a few days next time.
So, have you wanted to stalk me? Here are the locations, thanks to Google Earth, of some of the places I've talked about.
My hotel: lat 21.128243, long -86.764065
North beach, Isla Mujeres: lat 21.256785, long -86.750230
El Rey Ruins: lat 21.060824, long -86.781457
Chichen Itza: lat 20.682803, long -88.568630