Some good deals can be had for trips such as yours. Check Avis or Budget and ask for "Island Hopper Rates for Hawaii". Usually defined for car rentals of 7+ days and 2+ islands. Basically you pay the cheaper weekly rate but can use as daily car rentals on the different islands. Another possibility is an older link, but it may still be valid: http://www.onlyhawaii.com .

Forget the crazy-coupon rental-route. Most hotels in Hawaii have an arrangement that will add a car rental for an extra $20/day for an economy car. Call the hotels direct before your trip for details. One further tip. The roads in Hawaii are often narrow and can have many sharp turns. So this and with gasoline being $1.70+/gal, most people prefer the smaller sized cars. Enjoy your trip!