If you want something on the shorter side, we did the semi-sub excursion. It wasn't crazy early, and it would be really easy with a little one. We were led on a short walk from the ship to the ferry (literally a few min), then took a ferry out to the reefs and transferred onto the semi-sub. The ferry has a guide who talks about the famous people's mansions as you go past them, and he was pretty funny too. The semi-sub operator has gov't permission to go out on the reefs and knew what he was doing and talking about... and they tossed out food every so often to attract fish. We saw barracuda,
nerf shark, lots of colorful tropical fish, etc. When time was up, the semi-sub went back to the surface and we transferred back onto the ferry. There was really no talking that time, just a boat ride. They dropped us at the far end of the straw market so we had to walk through there to get back to the ship, maybe 10min. My son really wanted to see a shark in its native environment, but neither of our kids wanted to be off the ship for long - so it worked well for us. If you've been to
Disneyland, the semi-sub feels sort of like the Nemo sub, which my kids love - and that was one of the reasons I picked it. It's narrow, and there are seats along both sides; you're looking straight out the windows under water. The entire outing was only a couple of hours.