My vague memory (from about 1990) is that the areas around the hotels have LOTS of motor traffic involving drivers who have had mucho cerveza. Then, there are the wonderful sand dunes and beaches where no vehicles are allowed. A beach cruiser with five to seven speeds would be ideal for the beach area. I did not see any bike rentals while I was there, but I suspect some of the larger hotels may rent beach cruisers.
The beaches near the hotels used to allow motor traffic. You could be walking at the water's edge at midnight, and a jeep driven by a drunken frat boy would run you down...be sure to ask if those vehicles are still allowed, and keep your eyes open...have fun.