Don't let others minimize Oahu! We have been to Kauai, Maiu and Oahu. Oahu is our favorite and the JW is our favorite hotel there. First of all you can get to Waikiki from the JW in 1/2 hour during non rush hour times. Oahu is more than the Arizona Memorial, it is the North Shore, Waikiki, the best beach in Hawaii (Lanikai), Diamond Head, Hanauma Bay (World Class snorkeling), Greatest Variety of food and restaraunts on the islands in Honolulu.
We are beachers also, and the JW lagoons are uncrowded and peaceful.

The JW is on the leeward side of Oahu and more arid and less rainy than the rest of the island.

Kaui is beautiful and full of the activities that you want to do but, has anybody mentioned it rains alot? The Kaui Marriott is on Kalapaki Beah on which Nawiliwili Harbor resides a working port, water is not as clear and snorkeling is not that good, with all the cruise and container ships entering and leaving.

We disdain inter-island flights, you will loose at least 1/2 day of travel time.
You will have to return a rental car, take the shuttle to the airport, do the airport thing, then repeat the rental car shuffle. No thanks!