If on Maui, make sure to drive up Mount Haleakala. When we went, we drove through clouds until we were driving above them, stopped at an overlook where we were actually making shadows on top of the cloud within a circular rainbow, then drove to the top to watch the sunset. True story. Couldn't see the crater very well because of the clouds, but the sunset was brilliant. Just remember it gets cool up there.
Kauai is amazing, though. I'm a big hiking guy, so I would recommend the short/solid hike that goes to the top of the Waimea Canyon waterfall. Pretty sweet action there. Also, make sure to drive and see the Kalalau Valley Lookout. If you have the time, the Awa'awapuhi Trail to Nuopolo Trail loop is solid as far as getting a cool/lesser seen view of the Na Pali coast.
Lastly, try shave ice, and try it with ice cream in the middle. Both Halo Halo and Jojo's on Kauai are good. Lastly, a car is essential for Kauai and Maui. The only decent bus system is on Oahu.