I'll vote for Chichen Itza as well, and the flea markets are pretty cool too. If you want internet access, there's a pretty cheap internet cafe... more like a computer lab with a lan... that's pretty decent in the market. Your resort center should be able to tell you what water you can drink and what water you can't. Some of the tap water is potable, and some has bacteria in it. The beach is really nice there. The water is clear. Go parasailing if you're into that kinda thing, and "don't forget the tip, sir!"
Be prepare to spend a whole day out if you hire a guide to take you to ruins, but it's worth the trip. Also there's a snorkeling group that has a biweekly session. They go to a lagoon type place that has a lot of really cool-looking colorful fish. I'm not into snorkeling, so I didn't go, but I hear it's pretty cool.
Also, don't forget to wear sun screen. Kinda obvious, but I saw a lot of people get burned anyway for being retarded.
It's not hard to find good food while you're there.
There's a bull ring about half an hour from the resort center. If you want to see a bull fight, there's a good chance they'll have at least one while you're there. There are actually two bull rings... one main one downtown and then another by the market, if I remember right. Go to the one downtown.
That's all I can remember off the top of my head. Have fun... it's a great place to visit.