There are so many cenotes between Tulum & Playa del Carmen. I only go the opportunity to visit two of them and can't wait to go visit more! Same as any other touristy sites, the less known and the earlier you go the better if you want to avoid huge crowds.
All of these Cenotes will charge an admission fee, the price varies depends on how well these places are built and maintained - think of these Cenotes as ecological parks. I don't remember exact pricing I think it's about $10 USD per person for Gran Cenote and lockers & snorkel rentals cost extra. If you plan on going to a few cenotes it's best to buy your own snorkel to bring with you.
Do not apply sunscreen or bug screen few hours before entering the water! I think these water are still? so any chemical in the water will hurt the ecological system in these waters. Buy and use biodegradable sunscreen if you can!
Without further ado, here are some of my favorite cenotes: