Oahu:
Hanauma Bay Park for snorkeling. I used to take a couple of bags of frozen peas with me and once out in the water start dropping them. You become engulfed in fish as they eat the peas. I don't think you can do that anymore, or at least you don't want to get caught. Get there early in the morning, before all the tourists get there!
While on that side of the island, go to the Maui blowhole. A volcanic tube that water rushes into and then blows out the top. Good for an hour or so. There is also a blowhole there called the toilet bowl. Do not get in it for a ride, no matter what people tell you. You wanna see my scars?
Drive around the island to Kaneohe Bay, look at all the things on the way, stop for shave ice, look at the mountains. Then drive back to Wakiwaki by going over the middle of the island. A great way to spend the day.
As others have said, tour the pineapple plantation, drink the juice, eat lots of pineapple, bring some home.
Find a decent place to buy Hawaiian shirts, Hilo Hattie is good, but spendy.
Spend the day on the beach, you are married, not dead. Great eyeball liberty! Try to be discreet, the new Mrs. may not see the humor!
Go to the beach at night, preferably under a bright moon.
Go up to Diamond Head and look around.
Go to the USS Arizona memorial, try to be nice to the Japanese there. I wasn't, I should have been.
If so inclined, and have the time, visit Punchbowl National Cemetery. One of the most moving of all National cemeteries. It is inside an extinct volcano.
Then go to the Big Island.
bob