That is a pretty good time to go, it's not high season for tourism, so less crowded.
8-10 days, you could visit two islands. If you have never been there before, spend some time on Oahu and visit Pearl Harbor and Waikiki. I'd recommend you stay at a hotel in Waikiki. You can rent a car just for day trips from Waikiki locations, or rent for the whole time if you wish.
I'd recommend you visit Maui or the Big Island (Hawaii) as your other island. You'll want to rent a car for the whole visit on either of these islands. I'd probably advise you to visit Maui.
I'm sure you can find an all inclusive, but you'll likely enjoy finding restaurants on your own more.
The water is warm and enjoyable to swim in.
Aloha!
Doc