Been a long time since I've been there, but... There are ruins not too far from Cancun, you can rent a car and spend the day there. In what they call "downtown" they have outdoor shows that are quite interesting and it's all free. Also the "flea market", where you can get you some pretty decent buys if you bargain right.
There are a lot of activities to do on the beach, and prices used to vary according to the hotel where you are staying. Some are free with your vacation package, some are not.
Across there's the island of Belize and you can fly there for the day or overnight, I don't remember.
Iguanas go about freely like cats and dogs here, they walk on the sidewalks with you and hide in the ditches. When one appears out of nowhere at night, it's scary the first time when you don't know what it is! lol Same thing with lizards. If your hotel has rooms right on the beach and it's not an "inside" hotel with an elevator, odds are that you'll find at least one lizard in your room during your stay.
Stay away from the water, I mean don't drink it right from the tap. If you like salsa, hot there means FRIGGIN BURNING HOT. Way past what we call fireball here. Your eyes will cry, your nose will run and your mouth will burn a long time.
We ate breakfast at Dennys a lot because we kinda knew what we were ordering. If servers don't speak English well, and it happened a lot, what you ordered and what you got were not always the same thing. It's easier to just keep it and hope for the best than it is to try to explain that whatever they gave you is not what you ordered. They might take it back and bring you something else that you still didn't order. That happened too. lol And you'll come across this problem at "good" places too. However, hotel staff are really good in almost every hotel there, but you don't want to spend your whole vacation at the hotel either.
If you have US money, they'll love you. If you have pesos, they'll treat you like any other Mexican.
Hope you have a great time!
Missie