Road to Hana/7 pools rainforest hike. See about 20 waterfalls in one very full day adventure. (Bring mosquito repellent and rain gear.)
Snorkel the reefs of Kehei and Makena. Each one is different and they are filled with fish and turtles. Very calm clear water in the morning with easy beach access.
Bike ride down Haleakala. I'm not sure if a 6yr old is up for this. I didn't think It would be all that fun (wifey's idea) but it was a really great adventure with breathtaking views and photo ops.
Iao Valley Rainforest Hike. Pretty easy and very scenic. Go early as it does get crowded, especially on weekends.
'Iao Valley Rainforest Hike, Maui - Hiking trip | EveryTrail
Things I tend to pass on: Heli rides and cattle boat tours to Molokini. We did these on our first visit there and many still like em but they are expensive and underwhelming imo.