+1 for Shaka's suggestion.
Also, if you get a high ground clearance car - like a small Jeep - you CAN safely do the Road Trip to Hana - full circle. There's a "road" that is not recommended on the South side, my rental made me sign a waiver that if I damage the jeep for going on that road, I'm responsible.
So that scared me away.
Talking to many locals - they said boo - it's an easy drive for a jeep, just that you can't go fast.
+1 for jeep - getting to the various "hidden" beaches. B-e-a-u-t-i-f-u-l
I was there in Oct 2011 - gas was 4.54$/gal, and driving some 700 miles I had to fill-up at least 4 times (15-16 gals) plus a 1/4 tank the last day.
Compared to the Prius that would do nearly all that on one tank of 8 gal.