Buses:

It's very inexpensive (about US$0.50) and convenient to get around Cancun by bus. Not all buses are airconditioned.  Driver will make change.

Taxis:

Within, to and from the hotel zone, it can be very expensive.  Always ask how much you will be charged before the taxi trip starts.  You can negotiate the price, make a counter-offer.

From the airport to anywhere: non-negotiable US$40.00 or MX$400.00 (pay in any of the taxi stands inside the airport).  From anywhere to airport: feel free to negotiate, but expect about MXP$200.00 pesos.

Downtown cancun: cheap.  Make belive you are a local, don't let the taxi drivers screw you over.

Taxis will not issue receipts, except if you buy within the airport taxi stand.

Rent-a-Cars:

Its better to rent a car or a scooter if you plan to move around a lot, its cheaper than taxis.  Parking is a pain in the hotel zone.  When you buy gas, keep in mind that Mexican gas stations tend to overcharge (or undeliver) gas, i.e.: they tinker with the meter and charge you 10% more liters than whats actually delivered.

Bicycles, roller skates: in the hotel zone, there are some strips were you wont have any problem, but around "Punta Cancun" (northeastern point of the hotel zone) you can forget about it.  The same applies for kid-strollers.

Walking: sure, why not....