Tulum is a popular seaside Mayan ruin site, located about one hour south of Cancun. If you can afford to, I would highly recommend purchasing a package with one of the many tour companies in Cancun. My wife and I picked Best Day Tours and were picked up and dropped off at our resort in Cancun by an air-conditioned bus. The cost of our tickets was approximately US$40 each, which we thought was reasonable, considering the price included transportation, admission to Tulum, and a guided tour of the ruins.
The ride to Tulum seemed very long, as there was not much in the way of scenery once you left Cancun. The actual ruins are located a fair distance away from the parking lot. If you are the adventurous type (and you are wearing comfortable shoes), you can choose to walk back to the ruins. The walk is a long one (at least 20 minutes) and, if you are visiting the area during the summer, like we were, a very hot one. However, if you don't feel like walking or have small children with you, you are also given the opportunity to purchase tickets for the tram that runs back and forth between the ruins and the parking lot. Once you reach the ruins, prepare to be amazed. For someone who has never seen ancient ruins before (like me), the sight of the Mayan city is an awesome one. There are many buildings and temples throughout the area that are well-preserved, but the main temple along the seaside cliff is by far the most magnificent. Unfortunately, tourists are no longer allowed to climb up or go inside the temples. Our guide was quite long-winded and spent close to two hours taking us through the site and providing us with a plethora of historic information. Once the tour was finished, my wife and I spent another hour or so snapping pictures and doing a little sightseeing of our own.
If time allows, I would suggest taking some time to walk along the cliffs or climb down to the beach. Some of our best honeymoon pictures were taken in those areas. Keep in mind, however, that there is very, very little shade around the ruins, so be sure to bring plenty of water with you. My wife and I both regretted not bringing our swimsuits along with us. After several hours of walking around in the hot sun, the beach was starting to look very good to us (the water looked a little rough, though). All things considered, visiting Tulum was one of the highlights of our trip to Cancun, and if we are ever given the opportunity to go back, it is one site we would definitely visit again.