The Big Island of Hawaii has everything you are looking for :) It's got 11 of world's 13 climates so it is extremely diverse in terms of terrain and weather (you can snowboard and sunbathe in the same day!). It has a TON of activities to choose from including a visit to the most active volcano in the world.
There is one all-inclusive resort in the Hawaiian islands chain and it is located on the Big Island...Kona Village Resort: http://www.konavillage.com/ The only thing not included in the price is alcoholic beverages but includes all meals (they will even pack you a lunch and/or give you meal vouchers if you decide to venture off property...which you def should). Kona Village also has the best luau on the island on Friday nights...also considered the best luau of all the Hawaiian islands (we went TWICE last year on two separate visits because we loved it so much).
I would highly suggest taking a couple days in a B&B in the little town of Volcano on your trip...that area is at a higher elevation and in a rainforest so it gets chilly at night, really romantic in a room w/fireplace. Plus, seeing the volcano flow at night is amazing.
There are other really amazing hotels on the Big Island too, if you decide to not go all-inclusive. If you are looking to do many activities and not on hotel property that much then you'll probably get more bang for your buck staying at a regular hotel (I'm totally biased but LOVE the Mauna Lani Bay Hotel & Bungalows).
The next time you are at a bookstore, flip through the book Hawaii: The Big Island Revealed by Andrew Doughty. The Revealed series is considered the unofficial guide book to the islands. It has every activity available including vendor contacts plus hotel and restaurant reviews. We've been to the Big Island many, many times and I still take my Revealed book everytime and always manage to find something new.
Good luck!!
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