We went to Chetchniza (spelling?)with a tour group for one day to see the Maya ruins. We also ventured out on local and long-distance buses and ferries to Cozumel and Isla Mujeres, 2 islands easily reachable as day trips. Cancun is very commercialized and Isla Mujeres is much quieter and Mexcican. We do not speak Spanish but we got around just fine surrounded by the locals. We felt it would be more fun than joining a tour group.
We snorkeled, scuba dived, took day cruises, shopped at night market and on some days we had long breakfasts and then relaxed on the beach. Talking about scuba diving, we are not certified divers. And it takes a few days to get certified. So in Cozumel we negotiated with an outfitter a diving tour for us to this remote diving location. We had a blast! The underwater of the Cancun area is the 2nd largest reef in the world. If you only go sight seeing on land, you've only seen half of Cancun.
Other than the almost daily snorkelings on different beaches along the resort, we took a snorkeling tour near Isla Mujeres. We saw tons of colorful fishes (more than in Hawaii), plants, and even an underwater statute of Maria with fishes swimming around.
We did not plan on taking all the tours offered in the local commercials since we plan to go back. Enjoy your trip.