These are just my opinions, but it would be the Blue tang for me because the location is so great. You can take the water taxi to caulker from the pier near Fido's, it leaves at various times daily.
As to kayaking to shark/ray, I may be wrong, but I think you must have a guide, as it's a pay to use/protected area. You know, no fishing or taking of coral ect...
Cave Tubeing is a blast, I've done the Jaguar Paw tour & had a good time, usually includes the zoo as well.
If you have a day to give to a tour, I would suggest the trip to Lamanai with Tanisha Tours. This one takes you by boat up one of the rivers to the village of Bomba, there you pick up a bus and proceed to another river. The boat there takes you to the mayan ruins of Lamanai. It includes lunch, & seeing much of the interior of belize, as well as countless birds and wildlife, much that you miss if you stay on the caye.Enjoy! You're gonna love it.