We did the 15 night R-T on the Island Princess in January -- had a wonderful time.
First, I would highly recommend the Revealed Hawaii books -- there is one for each island. These were a God send to us, compared to Fodors.
* In Hilo (our favorite port), we rented a car (easy) and went to two water falls and Boiling Pots, and then to the other side of the island to Volcaanoes National Park. A *wonderful* day and lots to see. You can also stop back after Volcanoes at the Madamia place and Orchid store if you still have time.
* In Maui, our friends had rented a car -- lovely scenic drives, and lunch at Kimos in Lahaina I would say is a must
* In Kona, it's easy to do the town on foot and using the shuttle buses. Some things to see but a low-key port.
* In Kauia, again we rented a car and drove out to Kauai State Park (gorgeous) and all the way to Princeville. Again,. the Revealed books were our guide.
Hope this helps,