There is so much to see and do on each island and of course not enough time. In addition it is hard to know what someone else will like.
If you have two days in Oahu, use one of them to take an ALL DAY tour. Several operators run them and they will take you everywhere from Pearl Harbor to Diamond head , to Dole Plantation, North Shore, Pali lookout, Huanama Bay etc. Such tours can be booked in advance with American Express travel at Hyatt Regency in Honolulu .
If that is too much, just be sure to go out to Pearl Harbor early in the morning.
In Kuai , you will want to see Waimea Canyon WAilou River and Falls and the Emerald Grotto. Again an all day tour takes all this in. Another option is a boat ride or helicopter of the Napali coast line.Maui offers SOOOO much . We spent 6 days there recently and couldn't get it all in. To get the flavor, walk through Lahaina town , snorkel Molokini (May see whales and sea turtles on this trip) visit Iao Valley , tour the Maui Ocean Center, tour the Maui Plantation. Do the road to Hana , but this is an ALL DAY thing. Bicylcle down Haleakala Volcano. Kona is great place to go snorkelling. Try to get a zodiac boat ride along the coast to Cook's Lagoon,great snorkelling, great boat ride.
Hilo has everything from ManaLoa nut plantation, to volcano's , to hot springs along the coast, to orchid farms and more. From both Hilo or Kona you can do Helicopter tours of the island and volcano.
Want to try Luau? Try Paradise Cove on Oahu or Old Lahaina Luau in Maui.
Want a gourmet meal at a reasonable price, outdoors along Waikaki Beach with a beautiful sunset , try Hau Tree Lanaii. Located in the New Ohana Beach Resort.
(first Hotel past the zoo as you head towards Diamond head. Romantic setting under the Hau trees. Go for the Prixe Fixe meal you can't go wrong.