There are 7 major islands of Hawaii, but we mostly talk about 4: Oahu, Maui, Kauai, and the Big Island. Lanai and Molokai are very nice and open and quiet, but not as frequently traveled. Niihau is private. All the islands are substantially different. I would never advise someone that if they’d seen one Hawaii island, they’d seen them all. That being said, I wouldn’t want to do more than one island in a week – there’s just too much to see and do and changing hotels and everything else that goes along with getting to a different island eats into relaxation time too much, in my opinion.