I'll cast a vote for Cozumel. Been there diving several times & just love it. There are many 3 to 4 star all inclusives on that island that are nice. If you want suggestions, shoot me a PM.
A few upsides:
Some of the very best diving in the entire Caribbean. Specifically, wall/drift dives, which is a rare combination. It's easy to find wall dives or drift dives but usually not in the same place.
Nice, white sand beaches surrounding most of the island.
Cost, you get alot for your money.
Being on an island, you don't see some of the problems common on mainland MX.
If you want to hit Playa Del Carmen, it's a quick ferry ride.
Short travel time. Flight from ORD to Cancun is 3 hours + a 1 1/2 hr cab & ferry ride.
There are Mayan ruins nearby if that's your thing.
As with many islands, it's fun to rent scooters & just bomb around. No worries of getting lost. There are some cool bars on the cliffs & shores of the undeveloped, windward side of the island.
Cozumel has great deep sea fishing. After the day is over, throw a few bones to the boat crew & enjoy your catch with some beers on the beach around a bonfire.
Cruise ships frequent the harbor in town. If/when you get bored of the all inclusive activities, you can hit town & have some fun with the revolving door of the cruise ship crowds.
Plenty of other activities like dinner or booze cruises, sailing, para-sailing, horseback riding, ATV jungle tours, etc.