If you are in Cancun and want to escape the endless stream of 18 year old American kids getting hammered and vomiting in the streets and the other general tackiness of the city get on a boat and make the 30 minute journey to Isla Mujeres.
You will not believe you are only that far away from awful Cancun. It is absolute paradise here and one of the most beautiful islands I have ever visited.
There is a hostel there called Poc Na http://www.pocna.com which I have stayed in a few times now and its ace.
You can get a dorm bed there for about $10 aussie and its great.
Bit of a party place but you can also avoid that if you want to as well.
Isla Mujeres, Mexico
