Why on earth would you stay in Cancun when Playa del Carmen is just 30 to 45 minutes south, by bus. It is by far the coolest place I have ever been. Very european, very clean, and upscale. Hot Babes, and some what cheap. Cancun is a dirt hole compared to Playa.
I suggest you rent a jeep, and make the track south. Rent a hotel, in the city, and enjoy yourself. If you have time go check out some of the Chanute's, or drive down to Akumal and go to La Vida Loco, an old Grateful Dead hangout, with treehouse tables in the Palm trees over looking the beach. Xal Ha or something like it, is a major tourist trap that is worth going to if you have two weeks of time down there and need to fill in the gaps.
Oh Yeah, stay away from the Monkey's they bite.