I understand not wanting to move in the middle of your trip, but the flight it short, so its not too bad.
We've taken several trips each to Oahu, Maui and Kauai, and recently purchased at the Westin Villas on Maui.
Oahu has some quiet spots, but they are harder to find than on the other islands. Honolulu/Waikiki is pretty busy. Not NY or LA busy, but still.
Maui has a great variety of scenery. There's a dormant (not extinct) volcano, rain forests, beaches, snorkeling, etc. It also has some great restaurants and shopping, though much less than Oahu.
Kauai is beautiful, very green. I'd say it doesn't quite have the variety of scenery as Maui, and has less in the way of shopping/restaurants. But it is even quieter and less populated, and as I said, is simply beautiful.
Haven't made to the Big Island, but we do plan to. From what weve heard, you need to plan to do much more driving due to the size. Pretty quiet, and it does have the advantage of having an actively erupting volcano.
If you are really looking for the most quiet and laid back experience, I'd go with Kauai, though Maui does offer the advantage of being able to take day trips to Lanai or Molokai, whcih of course are the quietest of all.
If you want quiet and laid back, but still want a little "action", go with Maui. (Believe me, its all relative... by mainland standards, Maui and even Oahu are still very laid back.)