Xcaret is a great place for outdoor activity. It combines swimming, nature, and Mexican folklore all in one.
This was my second time to go to Xcaret. We really enjoyed swimming in these caves/rivers. Some of the caves were dark, some were lit up, and some were exposed; either way, it was an adventure.
Definitely bring snorkels; without them, you will get tired, as it is a lot of swimming. You can also float on innertubes in some area. After you are done, you will be treated a fantastic Mexican folklore show. I must say this was an awesome show! The production was almost Disney-like--great singers, great costumes, and they had a variety of themes but were mainly talking about Mexico and how it evolved until the present day, showing the rich and diverse culture.
You can either go with an arranged tour or go directly to Xcaret and buy your tickets there. We did the tour thing, which is basically transport there; there's really nothing to explain. Just swim, eat, and have fun; then go see the show, as we did!!
P.S. I know I said this many times--bring your waterproof camera!