There are countless cenotes along the Riviera Maya; fresh water sinkholes unique to the Yucatan peninsular.
I always recommend these three cenotes as they are cheap to visit, are as beautiful as any cenotes I have seen in the area and are easily accessible by public transport from Playa del Carmen.
They are located close together on the jungle side of the highway, about a forty minute colectivo ride from Playa.
See the transport page for more info on how to get here.
Cenote Azul is is my favourite. It’s open, filled with fish, has a ledge to jump from and is very picturesque.
Of course, they are all beautiful.
Cenote Cristalino is fairly similar, and Jardin del Eden is much bigger. There is a ten minute walk along a dirt path to Jardin del Eden from the highway, so in the heat of summer, it might be wise to go to one of the other ones which are closer to the highway.
Cenote Jardin del Eden (Garden of Eden)
Snorkeling gear and life jackets can be rented on-site, otherwise bring your own. Entrance to each of these cenotes is around 80 Pesos, or US$7.