The Road to Hana. Waterfalls, black sand and bamboo forest. Start early, and treat it as the whole day rather than a drive to somewhere.
Haleakala at sunrise. Above the clouds at 10,000 feet, and genuinely cold. Reserve it the day the window opens.
Whale watching. December through April, and Maui is the best island in Hawaii for it. The channel here is a nursery.
Molokini crater. A sunken volcanic rim off the south coast, and the clearest snorkeling on the island on a calm morning.
Upcountry and Makawao. Farms, lavender, paniolo town and a completely different Maui from the coast.
Iao Valley. Green, sheer and historically significant. Needs a reservation for out of state visitors.