Snorkeling- a friend who just got back from there did her favorite kid friendly spot was kapalua bay beach. But there are several spots to snorkle around the island. Here's a link to a page I've used to help planning
Haleakala National Park... looks cool to me, I'm excited to go.
Road to Hana- were planning to drive to Hana and spend s couple nights out there. There are some black sand and even red sand beaches we plan to check out.
We're also doing a luau (i know some people don't like them...but i do, so...) and a glass bottom boat so the littler kids can see some of the fish and reef since snorkeling isn't an option for them.
I'm still doing research... my friend lent me a book called maui revealed by i haven't gotten in to it very far yet.