Vacationseeker, please don't stay on the ship as someone suggested. If you are interested in Mayan ruins, then Progresso / Merida is the port for you. You can visit Chichen-Itza, which is one of the largest and most beautiful of the Mayan sites. You can't come close to seeing it all in a day but you can still see enough to appreciate and understand what it once was.
Uxmal is another that is not quite as large as Chichen, but is large and some say more beautiful than Chichen-Itza.
Both require a bus ride but to me that just gives the opportunity to see the countryside and see some of the real Mexico. Our bus was large, airconditioned, comfortable and had a bathroom. There were nice shops and restaurants at Chichen.
Even if you don't visit the ruins, do something. Than if you don't like it, you can at least say you were there, didn't like it, etc. If you stay on the ship, how will you really know ?? Have fun.
Ron is correct -if you want to see Mayan ruins this is one of the best ports.
Personally I prefer Uxmal (more Mayan) but it is usually easier to get to Chichen Itza because it is so well known. Chichen Itza has strong Toltec influences but is very impressive and extensive. Both Chichen Itza and Uxmal would involve longish bus rides but as Ron pointed out the buses are comfortable. Nearer to Progreso are the ruins at Dzbilchaltun (not as impressive or extensive as the other 2 but there is a nice Cenote to swim in and a good little museum).
We had the best adventure in Progresso...we rented a car and went into Merida. It wasn't too terribly far and really enjoyed it.
Merida is the capital of the Yucatan. The locals were incredibly friendly and not pushy like other mexican towns. In fact we did not see any tourists at all. It was great to get their local flavor. We ate, shopped, walked the city, went to the city center market, and met a very interesting man that showed us around. We still talk about our adventure, and have been on 2 cruises since.
If you want to do shopping in Mexico, do it in Merida. We went on a city tour and rodeo show (interesting, but not something I would want to do again) and our guide was very informative. The prices are much better than other ports, and if you want to buy a high quality hammock, this is the place to do it. Also, people we met on the ship went to a beach not too far off the cruise (1-2 miles, I'm guessing) and they had a lot of fun. Anyways, have a lot of fun on your cruise!!!