Something obscure but they'd probably like is hiking Waihee Ridge Trail. About 8 km round trip with an altitude gain of about 1500 feet. Go earlier in the morning before the clouds hit that side of the island. From this trail head they can continue driving to circle the volcano until they come back out at Kapalua, although it's a one lane track in a lot of places. Again, get the driving and hiking done in the morning, before the clouds and crowds.
http://www.reith.ca/gps/waihee.html
A picture below from my trek in January:
Also, at Kapalua, there is a fairly lengthy trail from Napili, through Kapalua and out into the hinterland.
Go past Hana and find the grave of Charles Lindbergh in a very obscure, hard to find but very pretty churchyard. Treasure hunt.
Tedeschi Winery . . . . Interesting drive.
If they can find someone else with a car who wants to team up, they can actually start hiking from the top of Haleakala at roughly 10,000 feet, go down into the crater and exit about 3,000 feet lower. Park cars at both ends.
Pull off the road near Chez Paul, about five miles from Lahaina, park under the trees and do some snorkelling. They'll figure out the place from the other people already there.
Watching a sunset in Lahaina, watching the ferry to Kauai leaving the port.
Cowperson