You don't indicate what types of activities you are interested in so it's hard to guess.
If you are into Mayan history and culture the tour to the Mayan site of Chichen-Itza is great. It's involves a ride a fairly long bus ride ( a/c with bathroom) but this gives you a good overall look at the countryside. The ruins themselves are really something to see. Shops, restaraunts, etc. outside the site. Uxmal is another Mayan site that is really good, not as large as Chichen-Itza but well worth the time and effort. As the other poster said, for a look at something different but interesting, think about the Merida tour.