Walking the Abel Tasman is an incredibly popular experience, but for a bit of an alternative spin, you can also kayak alongside the route of the track as the sea is generally calm all year long.
Numerous helicopter companies (most based in Lihu'e) offer up-in-the-air jaunts over waterfall-striped Mt Wai'ale'ale, the island's central shield volcano and one of the wettest spots on Earth, and the sheer-hewn sea cliffs of the Na Pali coast, accessible otherwise only by ocean kayak.