There are a lot of great things to to see in Hawaii--Maui, the volcanoes on the Big Island, the surfing on Oahu, the rain forests, the snorkeling near Kona, the canyon and the Na Pali Coast of Kauai--so it can be difficult to pick just one island or one place to stay in Hawaii. One solution to experiencing it all is a cruise.
They generally sail out of Oahu, so you can fly in and out of the capitol of the state. This also allows you to explore this most populous and cosmopolitan island of Hawaii and all it offers. Take at least a few days there, because there is a lot on Oahu, but don't be disappointed when you don't see it all. It will be there in a few years for a return trip.
The ships sail overnight to the different islands, and then allow you full-day trips on each island. Each boat will offer guided day trips, but a great way to go is rent a car once on shore, and get out and see the island, snorkel, sit on the beach, hit a restaurant or two, before you're due back on ship in the evening. Don't be late, because they will sail without you. Plus, dinner is free or cheep on the ship, and, depending on the cruise line you choose, the food is often very good.
My advice of things not to miss: go the the beaches and restaurants on Maui, get outside on Kauai, go snorkeling off the Kona coast on the big island, and don't miss the volcano (You don't get many chances in life to see an active volcano).
One thing that you get for free on a cruise ship is a great view of the volcanoes on the Big Island from the deck of your ship, depending on the weather. If the night is nice, and the volcano active, but not too active, the captain will steer the ship so you can stand on deck and enjoy a view of the orange lava running down the side of an active volcano and crashing into the ocean to make more Hawaiian real estate.
This is an extremely rare treat. Buy a drink, put child on your shoulders and allow yourself to be awed by the power and rage of nature for a few hours.
A Hawaiian cruise is highly recommended for Hawaiian first-timers. Each island has it's own distinct personality. And while Maui is often the most recommended island, many people don't like it because it's more touristy than other islands.
Oahu has great culture and surfing, but it also has a big city with crime. Kauai is gorgeous, but it also has a rain forest that can at times live up to it's name--rain. The big island has the most wide-open spaces, and it has volcanoes and the best snorkeling, but the beaches there aren't for spring break-style or bikini watching (that's Oahu).
By taking a cruise and seeing it all, you'll learn exactly which island fits you the best, and then you can do something that's a tremendous amount of fun … plan your return trip to the island(s) you like best.