Having gone through what you're trying to do, I settled on Isla Holbox, off the northern Yucatan Peninsula, about a 2 1/2 hour ride away from CUN, and also requiring a ferry crossing, and golf cart taxi to your hotel. Ironically, it'll take us as long to get from the left coast to either here or SXM, because of the 3rd leg via car service.
This place seems really laid back, and any activity off the island is a good 2 hours away by car. Looks to have many fine restaurants, and rooms run the gamut from $20 well over $500. Plenty of nice accommodations. Seems most people go for the few activities--especially whale shark trips, but the season is now over--reading, swimming, sunning, eating, and drinking
No building is over 3 stories tall, and the roads are hard packed sand. If you really want to kick back, this just might be the place.
I did have to spend many hours a day for 3 days to finally convince myself I really wanted this island as my SXM alternative.