Are you a scuba diver? If so, you've got no worries. It's one of my favorite places, as you can see from the Hubs I've written about it. If not, you are a bit more limited. I would definitely recommend doing some snorkeling. Even though you are looking down on the fish from above, instead of swimming amongst them, it's still pretty fantastic. Although it's been a couple of years since I've been there, during our last visit the Mayan ruins actually on the island were inaccessible. Over the years, hurricanes have destroyed the access roads and they haven't been rebuilt. But if that sort of thing interests you, a quick ferry ride across the strait will take you to Playa del Carmen on the mainland, which is a good jumping off point for tours on the Yucatan Peninsula. Actually, now that I think of it, a trip over to the Playa del Carmen is a pretty fun way to spend a day in and of itself. Otherwise, cruise ships stop in every week and there are a variety of touristy type things to separate the cruisers from their money.
If you speak Spanish, I would also spend some time just walking around. It's a relatively safe place, since they know that all they've got going for them is tourism. They don't tolerate anyone preying on the tourists. And once you get 3 or 4 blocks off the shore, you get into a little more traditional areas where the locals hang out. But be advised - you aren't going to get "traditional Mexico" here as it is geared for tourism and is pretty modernized.