We hired an 18 foot sailing yacht, complete with our own skipper, for the day from a local American resident who had been living in Cancun for 18 years. o earn money for his own sailing trips he would loan his private yacht to tourists - we met him one morning while walking along the beach and he simply started the conversation by saying "Do you like my yacht?". It cost each (there were six of us) and worth every penny. We had the entire day to go where we wanted, snorkel for as long as we wanted, and we even went to the guy's private yacht club where we had a lobster lunch for ! To this day it is one of my most memorable holiday experiences, and to think I had to be pursuaded by my friends to go on the trip!
Quick Tips:
Go to downtown Cancun for tequila and local crafts - it's a fraction of the price you'll pay in the tourist area. The local supermarkets in downtown are really well stocked and have everything you could want.
Best Way To Get Around:
Ask anyone who goes to Cancun - the best and cheapest way of getting around is by the bus, which travels regularly up and down the main road through Cancun. The bus drivers may try and overcharge you - but it's so cheap anyway! You may even be treated to the delights of a local busker!!
Alternatively, use the resort bicycles - we did this one day and had a great day out. Or take a stroll - my two friends and I walked often in the late afternoons into Cancun's tourist malls and also to downtown Cancun, where we felt very safe.