Was just down there for my honeymoon
you are in store for some fun my friend------YOU MUST go snorkeling and experience the coral reef (the second largest in the world), its the most incredible water and scenery there is
some tips:
1) Xel-Ha is nice---its a huge lagoon that goes out to the sea so its crazy cause you can see the different levels where the fresh and salt water meet, you'll see sting-rays etc-------Just stay away from teh buffet food (its the most revolting thing ever)
2) Another great snorkeling trip I cant remember the name of, I think its snorkeling something Paradise maybe-----a boat takes you out on the water 3 different times to different areas of the reef-------they make you lunch (which is actually really good). Defintely reccomend going on this tour also
3) dont buy your pesos at the airport or the hotel---walk down the street and you'll find many banks which will give you a much much better exchange rate. also buy your water at teh convinence stores------otherwise you'll get ripped off hardcore from the hotel and restuarants
4) stay away from the tourist salesman who sell trips in the airport and on the street---they're mad annoying and are shady
5) go to Tulum--but get the package deal with Xel-Ha since Tulum is small and you have more than enough time to go at teh end of the day----just make sure you walk down that stairway along the cliff and go swimming in that little beach ------there is nothing like swimming beautiful beach at the bottom of Mayan Ruins