Bike Haleakala. If not biking watch the sun rise at the top. It's one of the most beautiful views I have ever seen. There is a visitor center and the rare plant "Silver Sword" is found there.
Go on a pineapple or farm tour.
See Maui by helicopter.
Rent a convertible and drive the "Road to Hana". Bring your camera it is very scenic. With waterfalls and quaint places to stop and enjoy.
Buy touristy things with the name Maui on it. You will only find Maui paraphernalia on Maui.
Check out the cities Lahaina, Hana, and Kaanapali. Go to the Lei shopping district and watch how they make Lei's. Check out the different ones. You will be amazed at the artistry of Lei making. The flowers are amazing. Also make sure to have a Lunch Plate Special. Hawaiian Macaroni Salad is like you've never had. Ask the locals where to eat. They'll tell you were to go.
Take home chips or cookies made only in Maui. My favorite is Kookwees. The best shortbread I've ever had. Don't forget the local coffees. Bring home a few pounds.