We have done the trip to Chitzen Itza on a Carnival sponsored tour bus.
The ruins are very interesting, and we thoroughly enjoyed the excursion.
The trip took about 7 hours, so we were very close to sailing time when we arrived back at the ship. Maybe if you do the tour yourselves, you can cut some time off of the trip. The bus was air conditioned and very nice. It seems like lunch was included and they sold drinks for a $1 or so onboard the bus. We did make one stop at a little village which allowed you to shop in their market. The guide gave an informative talk on the way to the ruins. I have talked to friends and family and no one I know has done this trip on their own.
Ecstasy 12/12/09,Conquest 4/12/09, Conquest 4/19/09,Conquest 2/09, Pride 10/08, Ecstasy 1/08, Miracle 3/08, Miracle 4/08, Miracle 5/06, Conquest 12/08, Conquest 9/07, Conquest 4/07, Conquest 8/03, Destiny 5/05, Adv of Seas 5/05, Elation 12/04, Jubilee 4/03, Sensation 6/00, Celebration 5/93, Celebration 6/92,Holiday 5/92, Tropical 3/90, NCL--Windward, Skyward, Sunward