Gotta do either the sunrise or sunset at the top of Haleakala Volcano. We just drove up to the top by ourselves for the sunset and it was breathtaking. I think you can also book more of a "tour" type thing - take a bus up to the top, watch the sunrise, and then bike back down the volcano, if that suits your parents.
I second the "blue book" mention. Really helped us get the most out of Maui when we were there.
http://www.amazon.ca/Maui-Revealed-G.../dp/0971727937