Didn't knwo there WAS a howler monkey excusrion. We went on the cave tubing excursion and had a blast (although we did not book via the ship and got a much better value). The entire time we were walking through the jungle to get to the starting point of the cave tubing, the guide kept saying we might see monkeys... we never did.... we were bummed
Yes, I booked through Reggie Tours. You can find them online and they are very good. They also do Mayan ruin tours and snorkeling I think. Just google on Reggie Tours Belize, you can't miss them. Many regular posters on this site have used them too.
Well, Can't help with Honduras.. never been there... But for Grand Cayman, do Captain Marvins stingray city tour... (I think Dave already told you that)..But I highly recommend that. Its a hoot hearing big burly men scream like little girls when the stingray's bump into them. But its also someone you "have to do".
Ocho Rios is not my thing. We hate Jamacia. You can do Dunns River Falls but so does everyone else. You can do it more cheaply by yourself and justtaxi to the park form the port. Just be sure to arrange a round trip and DONT pay the taxi until he brings you back to the ship. Although there seems to be a lot of taxis waiting at the falls so it might not be hard to get one back to the ship.
Other than that, be prepare to say "No thank you I don't want me hair braided" about a million times. Its really quite annoying.
Cozumel has a lot of options. The best snorkeling and scuba you can imagine. You can't really go wrong with any water based activites while there. If you want to see Mayan ruins, you can, but that will take an entire day.