Gran Cenote is only 10 min drive from Tulum city center. It's an awesome beginners cenote as it is not large, easily accessible, not expensive. The water is relatively warm, you don't need a wetsuit. They have lockers, however, we had our waterproof pockets for valuable stuff that we swam with. You can either rent a snorkeling kit or bring your own to be able to see stalactites and stalagmites, and small fishes. We had our own swimming goggles and that was more than enough. They also have hammocks and areas for relaxation. Note, that you need to pay with cash almost all entrance fees in Mexico.