I have rented a wide variety of vehicles on Maui and have to admit that I have enjoyed convertibles and jeeps----but in the most recent trip (two weeks ago) I couldn't find a convertible at what I considered a reasonable rate. I kept on checking the various car rental sites and with their 800 number (I use Hertz Gold and National Emerald Club for the most part). I also bid a number of times (waiting a week between bids) via priceline, bidding up to $40 per day. I ended up with a full size car through Budget at $29.99 per day for a Nissan Maxima.

P.S. In my opinion the road to Hana is worth the ride, but make reservations at Mama's Fish House for dinner on the way home