My wife and I did the Tulum tour our first time in Cozumel. We both enjoyed our time, but it is a long tour. It involves a water taxi ride over to the mainland, then a bus ride to the site. It is a great opportunity for some "Kodak Moments", but it's also an active archeological site, so you can't climb all over the buildings. The cliffs are spectacular, and if you don't see them, you might as well not go at all. The downside is, between the bus and water taxi, you don't get to spend all that much time on the site itself. It's an expensive tour also (at least through Carnival), and I wouldn't recommend it for those travelling with children (go to Xel-ha instead, so says my 12 yr old)...
Tulum and Chichen Itza are the most impressive ruins, but both are on the mainland and make for quite a long day trip from Cozumel.
You may want to consider visiting San Gervasio ruins in Cozumel instead. They are much smaller - so don't have the same 'wow factor' - but nonetheless are very interesting and are situated in the north-centre of the island amongst jungle - plenty of opportunities to see iguanas, birds and other interesting wild life.
The other big advantage is you only need a few hours, so you could spend the rest of your time in Cozumel on the beach or shopping, rather than traveling on a bus!
To get there, you can hop in a taxi from the pier or downtown. It takes about 20 min each way and the driver will usually wait for you. I imagine it costs around US$50 (or less) one way in a taxi, but someone else may be able to say for sure.
If you're really into your history then hire one of the friendly official guides to give you the full low-down as you explore the ruins too...!
We did Tulum, just beautiful. Be sure to bring your bathing suits, they have the most gorgeous coast line there, it's so beautiful it's surreal. Did this tour with a combo 1/2 day including a stop at Xel Ha marine park, terrific snorkeling! My only wish is that we had more time there. Last year we went back to Cozumel and went on the Passion Island Beach thing, where they film the Corona Beer commericals, it was wasted from the hurricane that had blown through a few months prior. Have fun!
We did the Tulum / Xel Ha combo in 2003, and enjoyed it. Our bus stopped at a small store for a bathroom break, and we were able to buy some very nice locally hand made souvenirs. I regret not buying one of the handmade hammocks being sold by locals just prior to entering the area of the ruins. The history and beautiful beach at Tulum is well worth the trip, and my kids (early 20s) enjoyed Xel Ha much more than my husband and I did. We are going back on NOS this November, and look forward to returning to Tulum, but I'm looking at booking outside of RCCL's excursions because we would like to stay at the beach on Tulum. If you decide to go, I suggest that each member of the family take a bottle of water….it is very hot, and lots of walking.
Yeah, and to follow up with that, the RC Tulum/Xel Ha combo escursion is fast paced, I'd do one or the other. Both were great, but with all the traveling on the bus and boat to the mainland it doesn't give you enough time at either place to really enjoy them like you should. We are a younger couple hitting our 40's, my wife and I along with our 13 year old son really enjoyed Xel Ha, supurb snorkeling there. I think if I had to choose, I'd have to reccommend Tulum first, and in agreement with the previous post, I would make a whole day of it and plan on using the beach. The beach and rocky coastline are absolutely breathtaking.