Waimea canyon in Kauai is beautiful! We did the ship excursion which was fine but renting a car and going on your own might be better. There are also helicopter tours over the canyon. Did not do that but I bet it's amazing.
Also did the luau Kalamaku in Kauai- fun and amazing!
In Kona, we did a snorkel excursion through the ship. Big Island Snorkel. Highly recommend!