For the big Island I would recommend taking an excursion to the Volcano, it is a one of a kind experience!!! We went on the Lava Walk as well, which was amazing. I guess you do not always see lava though, so it is a hit or miss type of tour.
On Kauai I would highly recommend an Air tour with Air Ventures. They have a brand new six-passenger plane. Our tour was breath taking, by far the most beautiful scenery we had ever seen. Kauai is an amazing experience by air! We booked out tour off the web site www.airventureshawaii.com
and they provided free transportation from the ship!