Didn't like Oahu much - too crowded.
Maui is the schist! Whatever you want, pretty much, its got it. You want high luxury - go to one of the resorts in Wailea. Stayed at the Four Seasons there, and it rocks, but you could buy an entire new big wall rack for the price of a few days there!
West Maui has a much more remote feel to it. Nice little resorts and gusthouses. Lots of activities, snorkelling, diving, hiking and biking up and down Pu'u Kukui. Go see 'Iao's Needle. Cool feature, scene of a major massacre during one of the last tribal wars on Hawaii.
Drive the coast to Hana, and stay overnight there. It takes you through the rainforest side of the island. Massive bamboo groves, pines, waterfalls everywhere you look. Bike down Haleakala - a nearly 10,000 foot descent. See the sunrise from the summit. Go diving - may I recommend Maui Pro Diver? Good bunch of guys, the boat is never too crowded.
But most of all, chill out. Don't try to do too much. You can always come back again, you only get to do the honeymoon thing once (unless your screen name is D-......oh, never mind