I'll throw a couple out there for Maui, We went there about 10 yrs ago.
The road to hana we rented a harley for the ride. if you like to ride it adds to the trip (for me anyway)... Visit black sand beach when you get to hana. this trip will definately be an all day thing. Wouldn't hurt to pack a lunch. picnic at one of the stop offs. a few scenic waterfalls along the way.
look into booking a snorkelling trip to Molokini Crater. lots of tropical fish to view, enjoyable. i'd recommend it.
Luau. plenty of them to get into.
Went to the top of the Haleakala Volcano. they do have downhill bike ride tours from there, we didn't do that but could be fun.
We did a whale watch / dinner cruise, but i am guessing that's out of season.