A three-hour bus ride from Cancun (if memory serves), Chichen Itza is an impressive site for those interested in archeology (especially pre-Columbian). The site has several structures, the primary being the imposing step-pyramid. I don't believe that it's as large as the Great Pyramid of Egypt, but the climb is taxing nonetheless, and not for those who aren't physically up for the task. (I personally found the descent harder than the climb.) Digs are still in process, some open to the public at various times.
