I would vote Big Island. if lounging at the beach is not your priority you can do a day hiking in the volcano national park. My other favorite, although this was a little expensive, was the trip up to the top of Mauna Kea the old volcano / mountain that is now one of the top astronomical observation points. Was really spectacular. We took a guided trip that was costly. Its sort of a hairy drive if you go yourself and i don't know how far you are allowed to go by yourself (on top of that some rental car agreements have restrictions)