If you have a week or more on Kauai and you've never visited Pearl Harbor, the Arizona Memorial and U.S.S. Missouri, you may want to consider taking a day trip to Oahu. Catch the first flight out and head straight to the ArizonaMemorial, or you may risk long lines and an hours-long wait.
If you want to travel to Maui to experience the Road to Hana or to the Big Island to see Volcanoes National Park, there are few convenient nonstop flights; most lay over in Honolulu, so you'll have to plan an overnight.
The Superferry, a new interisland, high-speed ferry service, was scheduled to begin trips from Kauai to Oahu and back in summer 2007. But after protest from some Kauai residents, plans were delayed. The group went so far as to form a human blockade across the entrance of Nawiliwili Harbor, thwarting the ferry company's two attempts to enter. The company then decided it was unsafe to run a schedule to Kauai until further notice, and as of this writing the ferry has yet to begin service to the island.