I love Kauai but with only 9 nts. and having to fly out of Oahu, you would be wasting valuable beach/sightseeing/relaxation time, checking out, returning car, waiting in airports, checking in, getting another car. Spend that time seeing Oahu and max out your island time. All the Hawaiian islands are wonderful. I have not been to Oahu but would love to explore the northshore.

I spent a couple weeks in Hawaii, one week in Kauai (Princeville) and one week in Ohau (Waikiki). Both are great in their own way but I can't wait to return someday to Kauai. It rained a lot in Kauai, but regardless this is one of the most beautiful places on earth. And unlike ultra crowded Waikiki, you will have entire beaches to yourself. Out of all the islands, Kauai is my favorite even when it rains.

I've never seen a place as beautiful as Kauai.