Possible places to eat Thanksgiving dinner:
(in progress)

From http://cs.uhh.hawaii.edu/~jeschke/hawaii/bigisland/restaurants.html
Volcano House (Volcano, Big Island)
Elegant setting, good food, usually not crowded, and very convenient if you are visiting Volcanoes National Park. Also, not as expensive as you might think, considering that they are the sole concessionaire inside the park. Big Islanders: try the Thanksgiving buffet! $$$

From http://www.mindspring.com/~quimbymoy/hawaiip3.html
Roy's (Waikoloa, Big Island)
...
had dinner at Roy's in Waikoloa, a great place to eat (but unfortunately they were already booked up for Thanksgiving!).