Waihee Ridge Trail was my favorite thing.
https://www.strava.com/activities/940909549 Not far from the airport, up the Kahekili Highway, it is kind of hard to find, there are not many signs in Maui, but well worth it, when you get to the top you see helicopters flying below you!
Another good hike is the Lahaina Pali trail, it does not have the same rain forest feel, but at the top is a wind farm and you can see for miles, at a few points you can see the highway were you started and into the clear water below.
https://www.strava.com/activities/911778311 This hike is sometimes done with a drop off at the other end of the trail near Maalaea and pick up where we started, you would need two cars, or arrange some type of drop off and pick up, the end and start are not far apart, but not a good walk.
If you do the ride down Haleakala Summit, book early to get one of the tours that uses good mountain bikes or one that lets you ride on your own. We used Maui Mountain Cruisers, because they were the only ones available last minute, the bikes were beach cruiser types, with under inflated tires, and we were rushed the whole tour.
This is the company I was hoping to go with
http://www.mauibike.com/ My wife and I are avid scuba divers, we dove the Molokini crater, it was a good dive (my 500th!), but I don't think it would be great to snorkel, it might have been the conditions, but there was not much fish life, and it was below 30 feet. We could hear whales singing the whole time.
Another great thing to do, about a mile past Mama's fish house there is a surf beach, go down to the sand and walk back to the right, there are large sea turtles resting in the sand, you can get within 5 feet of them.