We pre-arranged a rental car in every port and did our own thing. It was awesome! Plus, we got to do what we wanted to do when we wanted to do it and got to do things and go places the excursions didn't or couldn't go. Oh, and I forgot to mention, IT WAS MUCH CHEAPER!!!
In Hilo, we got upgraded to a brand new Ford Mustang convertible with 12 miles on it. The cost was only $45!!! We couldn't believe our luck. And wouldn't you know it, it was either misty or rainy the whole day! Now that's my usual luck! But it was still awesome as we got to go places the tours don't go and really enjoyed the island.
We took our GPS with us and had a wonderful time in every port. Personally, it's the only way to see the islands and I highly recommend it to everyone!
One note; make sure you pre-book your cars ahead of time as in some ports, they will sell out. But check all the major car rental companies to find your best rate as you may qualify for something good based on AAA or AARP membership.
Also, alot of excursions can cost as much as $45 - $75 per person and you have to put up with a large group doing exactly what the excursion provides in a short period of time. You can rent a car much cheaper and spend as much or as little time as you want at each place, plus visiting places where the large buses can't go.