I've rented cars dozens of times in Cancun, Playa, and Cozumel without any problems. I've had my own vehiche down here for about 8 years so my information is out dated. I Usually will go to 4 or 5 different places to get the best price. Usually around $175 per week with all insurance. By comparing places you will quickly find out the going rate. I would check online now to find the best price. Even though you book a particular car online don't get your hopes up for getting the car you booked. Deal with it when you get down there and be flexable. I always like to pay cash for a rented car so I let the agent swipe my card but not charge anything to it because I don't like a hold being put on my card. When you return the car pay in cash and get the swiped card receipt back. Make sure to take photos of your can and mark down all nicks, scratches, and anything else you find. Check to see if the car has a jack, a spare, plates, and that all mirrors are there and not damaged. Make suree to put the same amout of gas back into the vehicle before you return it. Most likely your car won't have any gas in it when you pick it up so go directly to a Pemex and fill it up. Make sure to get a phone number where you can call in case the car breaks down. They are usually pretty good about that and have mechanics all over that they can call to fix it for you. If you have to have work done on the vehicle and pay for it yourself make sure to get a receipt and keep the old parts. The rental place should deduct that from your bill. You should be alright as you aren't going that far.
There are 2 great cenoted about 5 to 10 minutes outside Valladolid. Dzitnup and Samula. They are basically across the parking lot from each other. Cenote Zaci is in the town of Valladoild. Cenote Ik Kil is righ down the street from the ruins of Chichen Itza and well worth seeing along with the Balancanche caves.
You really need a couple of weeks to explore everything around the area but a lot can be squeezed into a couple of days. Don't forget to relax also. If you wander into the food plaza in the square and decide to have pizza, make sure to order sauce as that is an extra ingredient.
Have a great trip.