1) there are a lot of waves in the beaches in and near the hotel district in cancun but they aren't very pleasant because they have a LOT of rocks on the beaches that makes it hard to get in the water.
2) i reccomend going to chichen itza..it's honestly really far but its really interesting if you're into history and architecture. playa del carmen is a really beautiful beach area (bring extra pesos for an umbrella and get there early) and try to get a ferry to cozumel or isla mujeres. xel-ha and xcaret and xplor are all fun parks to go to but they are pretty pricey for non-residents of mexico. about $70-102 per person (and that's not in pesos)
3) if you are an experienced driver (really experienced) i strongly reccomend to rent a car because at the end of the trip the bills for taxi fares are going to be huge. and if you plan on taking the bus, be prepared for sardine-like seating and no air conditioning, let alone random bus stops and a lot of walking. take snack food and pack it into your checked bag or any carry-on bag because grocery shopping there is kind of tough thing for people who don't understand spanish. that way, you always have something to take to the beach or wherever else you may go. MOST IMPORTANTLY bring one or 2 backpacks just in case you need to carry anything because beach bags are extremely uncomfortable to carry around when they are full. bring a small to medium sized cooler because that will save you big-time at the beach (if not, just buy a styrofoam one from oxxo) and if you don't speak spanish you might want to bring a spanish-englishdictionary just in case.
4) restaurants..one thing. stick with the ones you know. food poisoning in random pueblos is never fun.
5) xel-ha, xcaret, and xplor are all like waterparks that are really cool and completely natural. there are underground rivers, huge swimmable aquariums and other activities. also try and stop by some cenotes..natural pools. you'll see them on the maps or just ask your concierge. also if you go to playa del carmen, ask the people around the dock for the man with the "Rubi" boat. he and his partner are wonderful for snorkeling adventures. its cheap, interactive, personal and totally worth it. our price for a family of 4 was roughly 15 usd per person.
6) knowing spanish is almost entirely quintessential but if you don't, it's not that big of a deal because cancun lives off of tourism so they're used to english speakers. be patient with them if they don't understand. put yourself in their shoes and take a deep breath. also, when going shopping, don't pay the sticker price (at the markets, not the supermarkets) try to haggle with the prices. the people of mexico are extremely friendly and are willing to help with any of your needs. but remember if you are going to drive, be calm very watchful and bring a map or prefferably a GPS device.
have fun on your trip. cancun in amazing.