Have done the stingray thing a coupla years ago and it was awesome. Chose not to this past cruise partly because we were told that Grand Cayman has "changed the rules" and that guests are no longer allowed to feed the stingrays. They used to give you a bucket of squid and the rays reacted by swarming in for free lunch. I wondered if it would be the same w/o the food so we opted to have the Lido deck for ourselves that day.
The river tubing safari in Jamaica takes about 4 hours total including the 45 minute ride up/down the mountain. Plenty of time for an adult beverage in the port area upon your return. You can find apoximate lengths of time for excirsions on carnivals site.