With all the big names like Cancun, Tulum, Playa del Carmen, the best spot might be one you've never heard of, and probably don't know how to pronounce correctly.
Pronounced HOLE-bosh (the X is sh in the Mayan language), Isla Holbox is what places such as Tulum used to be and is quickly becoming the next big dot on the map on the fly-into-Cancun Mayan Riviera.
Even one of my favorites, Isla Mujeres (which I still love!) is getting too busy and full of annoyingly bad and overpriced touristy restaurants. Holbox is about to fill that slot. Holbox does indeed cater to tourists so you have a lot of fun bars and restaurants, but it doesn't have that "tourist crazy hype feel" ... yet.
Holbox, also famous for swimming with whale sharks (amazingly cool if you haven't done this already!), has the clear turquoise water, lazy sandy streets, chilled out feel you've been looking for. Go on a budget or fancy it up in a nice resort, you'll quickly be thinking like Brigham Young "this is the place". Oops, sorry for the Mormon reference, I used to live in Park City. ;)
It's a bit harder to get there than say, to Isla Mujeres or Playa del Carmen, but it's pretty easy in the whole scheme of things. Take the ADO bus about 3+ hours North from Cancun, then a ferry out of Chiquila, a golf-cart taxi into town, and you're in your new paradise. Very few cars, lots of hammocks and cold drinks.
Budget Accommodation
There are a lot of places to stay, but here are some that I bookmarked for my next stay:
Starting at $23/night for a tiny bungalow with private bathroom not too far from the beach.
Starting at $56/night for 2 people in a large air conditioned suite with a balcony near the beach, or for a suite with kitchenette. The hotel also has a pool. Great value.
Choose between preset tents with mattresses and fans $23/night 2/pp, or little A-frames for $46/night 2/pp. Good location close to both beach and town.
Bright rooms close to the beach with mini-fridge and balcony for around $75/night.
Rooms with A/C, not far from the beach, bright, some with balcony starting at $30/night.
Simple rooms, some with private bath from $30-$50/night in the kiteboarding building. Close to beach.
A funky hostal with preset tents with comfy beds or dorm beds, but a bit further from the beach. $13 1/p, $18 2/pp