Credential: I've been to Hawaiin Islands 3 times, 40+ days of solid vacationing/trying new things every day (everywhere but Lanai)
Anyone that says the food in Hawaii isn't good, isn't doing it right. Check Yelp for restaurants.
Haleakala sunrise is a must. It's the most beautiful natural landscape I've ever seen. It feels like you're on Mars. Hike into the crater.
Obviously, road to Hana.
Drive all around West Maui through Kaanapali. Beautiful coastline/scenery.
Hiking is a must to see the islands: Waihee Ridge Trail is pretty nice. 'Iao Valley Rainforest hike.
Hawaii isn't the place for nightlife, so have some drinks, get to bed early, get up early, and go for hikes.
This was one of the best meals I've ever had:
http://www.yelp.com/biz/star-noodle-lahaina
If you're over on the Big Island, this is the BEST meal I've ever had:
http://www.yelp.com/biz/rapanui-island-cafe-kailua-kona
Incredible restaurant, very pricey though (~$100/pp):
http://www.yelp.com/biz/mamas-fish-house-paia-3