We did use our passports for our cruise, but they were not mandatory. My brother was able to use his birth certificate just as easy. The most important thing is to have a picture id or 2. The ship will check your picture id everytime you return to the ship. We seem to remember that at embarkation in Honolulu they asked for 2 forms of a picture id, but like I said, my brother did NOT have a passport and had no problems.
OOOH, the islands!! ;0) There are so many wonderful things to do on each island. My husband is a research hound when it comes to vacation. He gathered tons of useful information. We went a day early to Honolulu and spent some time snorkeling there. In Kauai we saw some wonderful waterfalls, a lighthouse, and hiked the Napali coast. In Maui, (we were there a little over a year ago) snorkeling, beach bumming, shopping, etc. In Kona, we did a raft snorkel adventure - this was definitely the highlight of our trip. We got to see and our boat was pretty much in the middle of a pod of 100 or so Spinner dolphins. WOW!!! In Hilo, we went on our own and got to see some (little known) great lava flows (1990) and were able to travel down a road that took us about a 1/4 mile from the active flowing lava. We did so many things, yet there were so many more we would have loved to do. Depending on your likes, these things may be great for you. If you have any more specific questions about each port, we would love to help you out!