It's FUCKING HAWAII, of course it's going to be expensive. Next time you go to Hawaii I know a real cheap place to eat. Costco, $1.50 for a hotdog and drink.
Oahu: north shore, diamond head, Waikiki, pearl habor, dole plantation, hanauma bay. Fok Yeun if you want incredible authentic Chinese food (right across from the convention center). Drop the wife off at ala moana shopping center and you'll have a day on the beach all to yourself.
Maui: Haleakala to see the sunrise and ride the bike down from 15,000 feet if you dare. Road to Hana will guarantee you to throw up. Wind surfing, whale watching, black sand beach.
Hawaii aka big island: volcanoes, and it's actually snowing over there so you can ski if you want. I don't care much for this island.
Kauai: not much here, perfect place to relax and hiking. Or you can spend a day on the golf course chasing chickens.
The most fun I ever had in Hawaii was on a scooter. This was on Oahu. A group of us rented scooters and ride around the island. It was sketchy because you have to watchout for the tourist drivers. But it was awesome. Oh yeah, you need to visit the Japanese Temple on Oahu too.