If you rent a car, make sure to eat the local food in the pueblo of Coba, the price is right and the food is authentic. El Bocadito Restaurant is the best restaurant around, service excellent traditional Mayan food and an astoundingly low price. El Bocadito also serves as the bus station and has a small hotel. After lunch, you can stop for a dip in one of the many cenotes on the road to Coba. Car Wash is a nice one.
Mayan pyramids are easy to climb, even with their steep crumbling stairs.
Getting to the top is worth the effort. However, many people get up and are too afraid to come down. If you get up a Mayan pyramid and can't get down, first try walking down while keeping your eyes only on the step just in front of you. If this doesn't work, you will have to suffer the indignity of bumping down the stairs on your butt. Don't be shy because you won't be the only one.