Looking for things to do with my own transprtation in Playa Del Carmen & Mayan Riviera without using tour serv

I have rented a car and have heard that you can snorkle and other excursions without using companies like 'altournative' or 'alltours'. When I was there last month I saw signs nailed to trees that read 'cenote'. Has any one ever been to one of the places or other ones like it?