Cenotes are sinkholes and natural wells were underground rivers come up to the surface. Most of them are basically natural, fresh water swimming pools inside a cave. Around Valladolid there are many cenotes you can visit and also swim in. Inside the cenotes it’s mostly humus and hot, but the water is refreshing. Most of the cenotes require wearing a life vest at all times. There is also some marine life, so bring (or rent) snorkels.
If you have a car you can visit quite some cenotes in one day. I would not recommend you to visit all the cenotes, as the experience is quite similar. Just do some research and find your favorites.