The best things to do are snorkeling and Xcaret (pronounced sh-karet).
Snorkeling in Cancun or Cozumel (which is an easy day trip) is some of the best in the world. The coral reef there is the second largest in the world. The colors and varieties of sea life were amazing.
Xcaret is a nature park. You can see all kinds of wildlife, take a snorkeling trip, swim at their beach, take an underwater walking tour, or go scuba diving. And the night show is not to be missed - it is a cultural show and is beyond words.
Quick Tips:
Remember the sun is much stronger here than in the US. And in a lot of the nature parks (e.g., Xel-Ha, Xcaret, etc.), sunscreen is prohibited. Just watch your exposure and you should be fine.
All-inclusive is great if you are planning on spending a lot of time at the resort. However, if you are planning several day trips or eating in the hotel zone, it can be wasteful.
Best Way To Get Around:
The cheapest way to get around is by bus. It is only approximately 60 cents (US). However, the bus is sometimes very crowded and is not air-conditioned. And hold on - they drive very fast with sudden stops.
Taxis are a great way to meet the local people and really learn a lot. We liked this means of transportation because of the interesting people we met.