We are headed to Cozumel, Belize City, Roatan and Georgetown in 2 weeks on the Norwegian Wind. We really want to see some ancient ruins, but since the tours are fairly pricey and offered on a couple of stops we would like some advice on which one might be the best for the money.
The tour prices are not outrageous for what you get. The tour to Tulum, for example, is a full-day tour that includes the ferry ride between Cozumel and Playa del Carmen, the bus between Playa del Carmen and Tulum, admission to the ruins, lunch, and the services of the guide. BTW, this tour is very interesting and definitely worth the price.
If you opt to take the tour to Tulum, be sure to bring a large (liter) bottle of water from the ship and drink it as you go through the day to prevent dehydration. The ruins are in an area that tends to be very hot (it was in the 90's when I was there, which was in the winter!).
If you are interested in a less expensive option, there's a tour on Cozumel that combines a Mexican Cultural Show with a visit to the San Gervasio ruins right on the island. The Mayan ruins at San Gervasio are not as extensive as the ruins at Tulum, but they are much more significant because the island of Cozumel is sacred in the Mayan religion.
Unfortunately, I have not visited your other ports of call so I can't advise you on excursions in those locations.
I have been to Chichen Itza and it is awesome. It is a really long day so you may not want to do it from a ship. I was on a 7 day vacation in Cancun when I went there. The area is rather large with many different buildings to see. It is much bigger than Tulum.
If you're at all interested in snorkeling, you might want to consider a tour that combines Tulum with Xel-Ha. This would be possible if your ship is dropping passengers in Playa del Carmen for shore tours. Tulum is interesting and beautiful, but not very big. I think you'd be hard pressed to spend more than 2 hours at the ruins.
Xel-Ha is a water park and it's feature attraction is a huge lagoon that is filled with tropical fish. We also saw stingrays and turtles there, plus plenty of iguanas sunning themselves on rocks. We spent about 4 hours at Xel-Ha and it wasn't enough!
I've heard wonderful things about Chichen-Itza, and if you're truly a history buff and interested in Mayan culture, this would be a great choice. But consider that the 3.5-hour bus trip each way might be grueling for what will be a relatively short visit to the ruins.
We did the Chichen Itza trip in one day on a cruise, and it is one of my most vivid memories of the entire cruise. It was a long day (we left the ship at 7AM) but I would not have missed it for the world. We had plenty of time to see the ruins on our own with a guidebook (did not take a tour with a guide). It did not see anything of Cancun, but have not regreted it for a minute. The bus ride was long, but we say a lot of countryside and it was air conditioned, so we napped on the way home. We were the last tour back to the ship.