strongly recommend xel-ha. really awesome place to check out and expect to spend your whole day on a trip there. it's a long driver there/back. you can do 1/2 tulum, 1/2 xel-ha when going there too if you want to knock out both at once. we did that trip, but after being at xel-ha we didn't want to leave so we stayed there longer and skipped tulum. i hear it's pretty neat too though.
also take a day trip to isla mujeres. it's like a 45 minute boat ride from cancun and you spend the day chilling on that island. it's really nice over there and you can rent golf carts and drive around. there are some spectacular views from there as well. and the boat rides are fun too with the staff on the boat and unlimited drinking. you can usually do snorkel trips on that trip too.
i did scuba there (my only time scuba diving) and there isn't that much to see, even though they take you out in this place that has a buncha statues. i've snorkeled in places and seen better stuff. but it was my first and only time scuba diving and it was freaking awesome regardless.
but in general, what you see snorkeling/scubaing in cancun isn't really that great compared to other places i've been. if you haven't done much snorkeling though, then it will still be amazing to you. snorkeling is kind of like pizza - even a bad place to snorkel is still great.
xcaret is also cool and a day trip type of thing, but if you only do 1 of the excursions that is a far drive away, i recommend xel-ha over that.
i did one of those "mayan adventure" things too which was kinda cool. it involved swimming in cenote's, a biking thing, and one other thing i forget. but tbh i'd skip that type of thing and do the other stuff. the whole cenote thing was not as cool as expected, simply because it's darker than swimming in the ocean, and there is hardly anything to see in the water anyways.
and the clubs are ridiculous there. coco bongo, daddyo's, the city, etc. those are all awesome places to go. they are all a flat fee to get in and it includes all drinks inside so you can go pretty cheap at those. i don't recommend senor frogs, unless you are looking for an american bar in mexico. also eat at the little taco stands and stuff, those are cheap and awesome food.