Well, Hilo is your port for a helicopter tour of the volcano. We did one and it was fantastic. I was blessed with a window seat. You don't pick your seats; you are balanced by weight. Do check to see how the lava is running as close to the time as possible. There wasn't much, but enough, when we were there in 2008.
In Kauai, we did a movie tour, since it has been the setting of many movies. They took you around in a small bus where as you journeyed to each destination they would show a movie clip that was filmed there. My personal favorite was the scene with Lt. Cable in South Pacific. The place looks just the same.
They also took us to the Coca Palms Resort where Elvis filmed Blue Hawaii. Hurricane Iniki blew through it in 1992 and it's remained in that state ever since. Totally spooky and fascinating. There were still keys at the front desk!
In Maui, we did a rainforest hike. It was only OK. Sights were not that grand and lunch was crappy. I did enjoy swimming under the waterfall and the Japanese water shoes were a hoot. Made your feet look like a pigs hoof, but they were very effective on slippery rocks. It did rain, but duh...