Cancun 自遊行

I agree Playa del Carmen is better place to stay Cancun.  It is cheaper with the same nice beach. I remember there are local tours from there to Chichen Itaz, Tulum, etc.
http://www.playadelcarmentours.com/ (only for reference)

If you are certified diver, Cozumel is the palce to you.  I fnot, it is not worth it to go all the way to Cozumel for discovery scuba.  YOu can do it in Cancun or Playa del Carmen.