In Kauai we are staying at the Kauai Marriott. We will only be there 4 nights and 3 days, so 2 days of relaxing (we'll be tired from the wedding!) and 1 day we'll drive to Waimea canyon and to Poipu. I already have a reservation one night at The Beach House.
In Maui we are staying at the Westin on Kaanapali Beach. I already booked us 3 months ago to go to the Old Lahaina Luau. When we arrive we'll also book a trip on a catamaran to Molokini. I love to snorkel (and am getting my fiance into it too) so we'll also try snorkeling at Black Rock by the Sheraton as everyone on here recommends it. One day we'll do the drive to Hana. We'll also drive one morning to see the sunrise at Mt. Halekala.
On Oahu, we're staying at the Outrigger Waikiki. I think one day we'll do the Circle Island Tour (Pearl Harbor, Dole Pineapple, etc) and the other 2 days we'll relax.