We didn't use a car... the busses and taxis go everywhere you need (and cheaply).
Whatever you do, GO TO CHICHEN ITZA!. Probably the most amazing place I have ever seen... (Of course I am a history nut)
On a side note, All water in Cancun is purified, but if you go outside of Cancun, only eat and drink pre-packaged stuff! We ate some food at a resturaunt at Chichen Itza and got the worse stomach pain I have ever had in my life. Sooo.... if you do go to Chichen Itza, only eat and drink pre-packaged food... seriously... don't forget it!