Going to Maui, what are fun excursions for a 3.5 yr old?

We went snorkeling in the Molokini crater. It was fun, but it's really obvious that so many people and boats are there all the time, there's not as many fish or coral or other sea creatures as there used to be. I loved the aquarium and I bet a 3.5 year old would too. The Iao Valley is fun for short hikes, but it rains (mostly light rain) there a lot, so be prepared for that. We didn't get up to the north shore much, so I'm not sure what all is out there. How about Coral Bay? We didn't go, but it sounds like a fun place for kids.

Day hiking in Haleakala is fun. There's a great day hike on the way there but I simply can't remember where off hand. You hike through some cool bamboo, across bridges, and end up at a waterfall. It's just at some pull out off the road.

Dragon's Teeth at Kapalua's a fun little hike.

Next time, we plan to stay a couple nights at the big hotel at the far end of Wailea-cannot recall the name, but it's the big newish one. It has kid programs if you want to go kayaking and let him stay at the hotel. Or, we've snuck on before and enjoyed their little toddler pools, etc.

How about the pineapple train? That's a huge hit.

Aquarium sounds fun but I don't know anything about it.

PS: Oh, and there are some great little hikes when you're driving back from Haleakala towards Kapalua. It looks like you're cutting through cow pastures down to the water. But, there are a couple of big pullouts. One of them goes down to some really cool lava pools. We loved swimming in those last time.