Ah yes, Hawaii ... some good honeymoon memories. You really should try to visit as many of the islands as you can.
On Kauai, the Na Pali coast on the northwest side is quite spectacular, with steep cliffs going right down to the water, and gazing down into Waimea Canyon can make you feel pretty small. If you opt for a helicopter tour, take a Ziplock barf bag. And that's all I have to say about that ...
Maui is a great island, good amenities but a LOT less crowded than Oahu. Ed's right, you need to see Haleakala Volcano at dawn, but don't take the bike trip back down unless your life insurance is paid off. Likewise, if you want to flirt with death in paradise, take the Road to Hana. Just remember to watch out for the locals - they like to drive in the middle of the road and guard rails seem to be optional. I would skip snorkeling or diving at Molokini, because after the Navy detonated some old WW II-era bombs in place, there's not a lot left to see.
If you want something different on Oahu, rent a car and head out for the North Shore - a lot fewer people and some great snorkeling, and Haliewa is a delightful throwback to a bygone era. The Polynesian Cultural Center can make for a nice, although very pricey, day trip; you get to see the various cultures, true, but the atmosphere is vaguely Disney-ish. Do not try the green plantain bananas unless you want a memorable object lesson on eating strange foods from foreign cultures .
Enjoy!