I went to Maui and Kauai for my honeymoon and spent a week on each island. We are not much for just laying around on the beach, and there is a ton of cool stuff to see and places to go explore.
We had this guidebook which we found to be excellent, you might see if you can get a copy before you go -
http://www.hawaiirevealed.com/books-apps/maui-revealedThe trip up to see the Haleakala Sunrise and the bike ride down was a great time. Road to Hana and Up Country, dinner at Mama's Fish House, and a ton of other great memeories come to mind.
Have a great time