My 3 yo loved the Seaworld Explorer boat ride. They take you out to the shipwreck by another boat (which gives you a nice view of Palm Beach) and then transfer you to a semi submerged sub. The sub then goes around the shipwreck and there are a ton of fish (including the "Dory" kind from Finding Nemo). They let the kids eat on the boat. My daughter had a cookie and was waving it around the window (not intentionally) while she ate it and the fish swarmed her window. Everyone thought it was great! Since the boat doesn't submerge, if the kids don't like it you can go back up to the deck.

We also took her to the Butterfly Farm but she was bored with that. She liked just being in the pool and ocean and digging in the sand the best.