During various trips to Hawaii in the past to attend bonsai conventions, I found the time to explore each island and to note its specific geography and gain a general understanding of the whole chain's giology. In persuit of this I found renting helicopters to be a wise investment, as the best of what defines Hawaii exists on its higher flanks and remote river valleys unobtainable by foot or road.
A good rule turned out to be, "the smaller the helicopter the better". Although this often meant being cramped, it also meant the pilot could take you to just about anything you could poin at and being able to follow a waterfall all the way down from its misty source through its multileveled pools and finally to hover over its churning caldren at its base, on Maui http://www.vlbanting.com/waterfalls.htm and skimming over the sulfurous fumes emitted from fissures and craters on the "The Big Island".
A road was available on Maui that took one on a slow-motion action ride (yes such a thing does exist) along its coast which included 800 separate curves and 400 bridges. I rented a red Nissan 300 X for that drive although I didn't need it because one could never get up enough speed on that road to put it through its paces but it sure looked sporty against the backdrop of all that green tropical vegetation.