Nakele Point on the Northwest side of the island. Excellent hiking and a blowhole that sprays ocean water up to 70 ft in the air during the winter months. A great place to see just how vicious the ocean can be, crashing against the cliffs.
Haleakala Crater. The dormant island volcano that formed Maui. Viewing the sunrise is a highlight, although I've never done it because waking up at our place in Ka'anapali at 3 am to drive up never appealed to me.
The observatory at Haleakala's 10,000 ft. summit. One of the telescopes is used by the military to track space junk.
Sundown in Lahaina, an old whalers village turned into a cultural center and shopping/eating area.
The banyan tree in Lahania that takes up about two acres. It looks like a lot of separate trees, but in reality it's just one. Coincidentally, Jerry Garcia spent lots of time in this square painting the tree.
Whale watching. Highly recommended to everyone. It's really more like "whale waiting" because once you see the tail, they're diving for at least 7 minutes or so.
And a short stop at Planet Hollywood Hotel in Las Vegas to break up the plane flights. Maui was much more fun.