In Ocho Rios, I highly recommend the shore excursion that combines a tour of Prospect Plantation with the visit to Dunn's River Falls. Prospect Plantation is really fascinating. The terrain is quite rugged (Jamaica's Blue Ridge, which exceeds 6300 feet above sea level, is higher than Mount Washington!), so there's quite a science to the way in which they plant crops in various altitude zones. Be sure to wear your swimsuit under your street clothes and to brng along a towel from the ship and a pair of Surfwalkers (available at most major sporting goods stores) to climb the Dunn's River Falls, too! It's not a particularly difficult climb, so long as you have the right shoes. One woman who did it was 99 years old. The tour also includes a brief shopping stop, where you might want to pick up some Jamaican Blue Mountain Coffee since it's a great souvenir for your coffee-drinking friends.
In Cozumel, I could go either way with the trip to Tulum. It's a fascinating trip, but it does consume the whole day. If you opt to take that trip, be sure that each person in your party has a large bottle of water and drinks regularly as you tour the ruins, as the area tends to be quite warm (90's) even in the middle of winter.
If you opt not to go to Tulum, the tour which combines the Mexican Folkloric Show in San Miguel de Cozumel with a visit to the Mayan ruins at San Gervasio is a great alternative that consumes much less of the day. The ruins at San Gervasio are not anywhere near as extensive as the ruins at Tulum, but they are much more significant because the island of Cozumel is a holy place in the Mayan religion.