Dolphins
At the Hilton Waikaloa Village on the big island, your kid can join in on the interactive Dolphin Quest program. This is a natural habitat for the smartest of the aquatic animals.
Horses
At the Makena Stables on Maui, kids can learn the lay of the land from the top of a horse. This isn't for the littlest ones, though; make sure they're at least 13.
Caves
In northern Kauai, there are the Waikanaloa Wet Caves. These caves were formed thousands of years ago by water pounding on the soft lava to make these forms. Be careful, as no swimming is allowed here.
Water
Maybe your kid isn't ready to hang ten yet, but there are other things to do. Opportunities for swimming, snorkeling, paddling and boogie-boarding abound. Get in touch with the local tourism office and they'll point the way to the nearest spot.
Hiking
Diamond Head Crater on Oahu makes for an incredible destination, as this short hike (30 minutes) up the extinct volcano gives a great panoramic view from the top.