Driving around the islands is easy enough. We did it when we went to Hawaii several years ago, (though not on a cruise). One caution: if you do your own touring around, by rental car, taxi or whatever, you need to be careful you watch your time. If you miss the ship's departure from the port, it will be up to you to get to the next one at your expense. If you book a shore excursion through the ship they will wait for you if your group is late.
I don't recommend a rental car on Oahu unless you are staying there several days before or after the cruise and you want to see the rest of the island away from Honolulu. HOwever, if you have the chance to visit the Polynesian Cultural Center on Oahu, don't miss it! You can do it by rental car or by bus trip.
And definitely check out a Hilo Hattie's shop somewhere in the islands. Lots of great souvenirs & gifts.