Hello and ALOHA!
I am an Islander, but Oahu. Oahu, as you know, has Waikiki Beach, Diamondhead, hotels all lined up along Waikiki Beach, and loads of people!
The Big Island of Hawaii would be your best choise! Volcanos, and Hawaii's tallest mountains! And IF you come at certain days of the "winter' months here, you can experience snow on the tropics! And the forest researve there is second to none!
Akaka Falls is Hawaii's tallest, and you can also experience "Boiling Pots", and others.
If you want the most picturesque island, you should go to Kauai! That is where they filmed Jurasic Park. And Kauai has a lot of water falls, and not to mention, the Grand Canyon of the Pacific: Waimea Canyon. I would truthfully avoid Oahu. REALLY not much to see and enjoy here exept shopping and the night life. And what you see on the post cards and travel videos, are what you see! To take IN Hawaii, you should visit The Big Island, then Kauai, then Maui! But on the Big Island, you will be driving for miles seeing NOTHING but black lava fields! But when you reach your destination, he black lava fields will REALLY bring up the color of the rain forest, the plush green tropical paradise you can only DREAM of! And you are "hearing" this from an "Oahuan".