JMQRSQ

We are cruising Pride of America in February and I am wondering about getting a rental car when we are in Hilo. We are there from 8am to 6pm and the ship offers an 8 1/2 hour excursion that offers most of what we want to see, but not all. I am wondering if anyone has experience renting a car. Is it easy to do? Would we have enough time to rent car and explore and get back to ship? It makes me nervous to rent a car and not stay with an excursion...