I live in Mexico and visit Cancun a lot. Whilst it's a stunning place to visit, when you consider how much things generally cost in Mexico, the prices charged for tours / restaurants / attractions etc are utterly ludicrous. Be prepared for virtually every person you encounter to have the sole aim of getting as much money out of you as possible, and definitely avoid any tours sold by agents in the street / airport / hotel etc. Airbnb can be a pretty good place to find excellent tours that won't rip you off: they don't just do holiday rentals nowadays. Friends have had decent success with TripAdvisor too. Neither of these will be cheap, but they probably won't cost more than anything you'd find when you're there, and they're a lot safer and more regulated.
If you're a confident driver, I'd hire a car and visit Chichen Itza, ideally at night for the "noches de kukulkan" light show which is stunning. You'll get a great view of the stars too. The car will allow you to visit some of the cenotes which are a little more off the beaten track, which is a far more authentic experience than the ones the tours tend to go to. The Tulum ruins are great too, and there's a beautiful beach nearby which is well worth a visit. Beyond that, the main things to do in Cancun are adventure parks, boat trips etc. When I go, I tend to book a nice hotel on the beach and spend most of my time there: if you want to really get to know Mexico, Cancun isn't really any good, so you might as well go for the "all inclusive" American style experience.
The guy above who mentioned the "Off" mosquito repellent is spot on: I have about 20 bottles of it around the house and it's by far the best I've used.