There are many people who successfully use Priceline for car rentals on the Big Island.
I've detailed basically the three major areas that are prohibited by car rental agencies on the Big Island in an earlier post:
http://www.flyertalk.com/travel/fttr...ML/000276.html
Basically, you cannot drive your car on any road not paved, including Saddle Road (which is paved but a very dangerous road), into Waipio Valley and South Point Road. Although many of the roads of Volcano National Park are paved, many also have been eaten up by lava. These roads are closed to the public and no access is allowed. There are other roads to access beaches such as the black and green sand beaches where 4 wheel drive will be necessary. There is also a link to a list of beaches in that original post.
I hope this helps. Aloha
EDIT: Sorry... didn't realize that the moderator had moved this thread from another board... seems like you have two of the same question on the Hawai'i board now. Well, hope this helps any ways
[This message has been edited by slippahs (edited 06-14-2003).]