I will be going to Cancun for my first time next week for my anniversary. We were thinking of going ATV, maybe horseback riding and seeing the Chichen Itza. Is it possible to go snorkeling on our own or do we have to join a tour group? Is the ocean clam enough to snorkel in? I welcome any other suggestions for activities?? =) Lastly, should I book in advance (online) or wait till I get there to book? Thank you in advance!