Sign up for a party package. When we went 2 years ago, we got a 5 night package. Every night a different club, except for one which was a booze cruise to some island where they held a wet t-shirt contest. I also suggest visiting downtown Cancun which is not as "touristy" as the riviera, with your friends. Avoid bringing valuables otherwise you'll be target for theives. Don't be afraid to try local cuisine (their quesidillas are AMAZING) but be sure to drink bottled water. We also went on a jet skiing tour which included snorkeling around the reef area.
As for price, I'll have to check with my friends who booked the party package for us but I can tell you, they're definitely worth it. Oh and be sure to follow the Girls Gone Wild crew because they're guaranteed to get the girls flashing.
