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.