Howdy,
If your 5 year old likes fish, the glass bottom boat is pretty good. The boat is large enough they can move around a little, then you go down below and can see lots of tropical fish, easy for small kids to do. They even had a complimentary fruit punch you can have on the way back. Restrooms are on board too. I've heard the gardens and zoo is nice for little ones too. No matter what you do you will have a short walk to every excursion which is ok in the morning, but after excursions kids are tired and want to be carried so you may want to take a collaspable stroller (umbrella type works great) for the way back.
Have fun!
mom x4,grandma x4