>>1954637When are you going and what island?
The best /out/ island is Kauai hands down. It's the most mountainous, has the most spectacular scenery and great beaches. Much more rural than Maui and Oahu though. You can spend 3 weeks in Kauai and still not see everything even if you move fast. North shore and south shore are like two different worlds. The snorkeling here isn't as good as big island BTW.
Oahu is if you like Las Vegas: tropical island edition. Some mountains. North Shore is beautiful but it's all super crowded. Tons of city party life but traffic forever.
Maui if you want a tropical beach getaway. Lots of waterfalls but not much mountains. Mainly hills and a volcano.
Big Island is really just a giant volcano and some hills/mountains/beaches. It's very big but super rural as well.