Kauai. I hear Maui and Kona are really awesome too, of course I'm sure all the islands are pretty awesome (perhaps with the exception of the overcrowded Waikiki). I'm sure you'll have a blast when you guys go next year. My advice, rent some snorkeling gear! At least for a day or two.

Lost eh? Lots of things have been filmed on the island, Jurassic Park, South Pacific, Gilligan's Island, the opening scene of Raiders of the Lost Ark (fleeing the natives on the river), the opening credits of Fantasy Island and a whole list of other things I forgot.