paddling through "cenotes" (they are at least partially man-made cenotes), and driving in amphibious jeeps. I've heard the other X parks aren't as good.
One tip: if you're going to buy tchotchkes, do it at the bigger stores as opposed to the street vendors. It's all the same stuff but the street vendors just try to haggle with you and charge really high prices, whereas the big stores don't haggle and are (relatively) reasonably priced.