You can't leave Oahu and not stop by the Dole Plantation for a Dole Whip Pineapple Split. It's absolutely pineapple-gasmic!
And I'd recommend
Germaine's Luau for your luau fun. They're the most fun, have by far the best food, and best capture the family style feast that luaus are supposed to be. It's so awesome that we even modeled our wedding after them! A drive up to the North Shore is also one of those can't miss experiences. We rent a car and do the circle island drive. The views are simply breathtaking. Oh, and stop by
Leonard's Bakery for malasadas! Way better than donuts, and you can even get them filled with haupia, which I personally don't care for, but is part of the Hawai'i experience. Also, to really experience the swank that is tiki, visit the
Thor Stor in Waikiki. We love to stop in and gawk at all the awesome stuff and pick up a new tiki mug. You'll totally get a kick out of it. And right next door is the
International Marketplace, which is an awesome place to hang out, eat some Dole Whip (though it's not as good as the plantation's, for some reason) and check out mega-kitschy market type stuff. If you want to pick up some aloha wear, stop by
King's Village. There is a store there that is bursting with aloha stuff for way cheaper than anywhere else, and it's good stuff.
I'll post more if I think of it. I'm so jealous! I love Hawai'i!