We rented a car in every port we visited in Hawaii. It's very easy - shop around to see who'll give you the best rate. In one port, we got a car for $25 for the day, but we didn't pay more than about $45 in any port except Oahu. We learned it's MUCH cheaper to book at the airport than anywhere else.
However, one word of caution; make sure you pre-book your rental cars ahead of time. In a couple of ports, all the cars had been reserved, so those looking to rent one were out of luck. Because we made all of our reservations ahead of time, we had no problems whatsoever.
We also took our GPS with us and it was very helpful getting us everywhere we wanted to go on every island. We're so glad we took it.