I have been to Maui several times, usually staying on West Maui...most frequently in Kapalua. I don't know much around Kahului other than the airport, a smallish mall, a Costco, and a Walmart.
I enjoyed the one time I snorkeled off of Molokini, but I hear it has gotten pretty congested with tourists. Lately, I just snorkel in Kapalua Bay.
There are zip lines and a lot of great hiking trails up in Kapalua. I hear they've also added zip lines in Upcountry. Haven't done any of the zip lines yet, but I love the hikes. I also had fun on ATVs in Upcountry...the one I did takes you to a spot where you can hike and see the rain forest.
I've gone up to Haleakala, but haven't done sunrise there (which everyone says is spectacular) or the bike ride down. There's also hiking from the top down to the Hana side, but I haven't done that yet.
Road to Hana is a must at least once (I've done it a few times...including running 9 miles of a relay along the 52 mile route), but there really is nothing to do in Hana itself. Past Hana, the Seven Sacred Pools are worth seeing...more hiking.
As for food, Mama's Fish House is awesome, but expensive. I'm a really big fan of The Banyan Tree at The Ritz-Carlton, Kapalua as well and, in the same neighborhood as the Ritz, Sansei Seafood Restaurant and Sushi Bar and Merriman's.
You must get one breakfast (specifically the banana - mac nut - pineapple pancakes) at The Gazebo in Napili Bay.
Try Ono Gelato in Paia or Lahaina...their tropical fruit flavors are amazing.
For ideas of what to expect, just click on any of the Maui links listed here:
http://akira3d.smugmug.com/travel