If you are going to Maui try to take a day and go to Haleakala National Park. 10,000+ feet above sea level, it is a volcano (no flowing lava). You can hike down into the crater.
This is the park everyone goes to see the sun rise, if you don't feel like getting up at 3 a.m. and standing elbo to elbo with thousands of people, spend the day hiking and go to the summit in the evening and watch the sun set, (same view only you are facing west).
Like others have said, the road to Hana is well worth a day, there are gardens, black sand and lava beaches, rain forests.
Also if you get a chance take a ride around the western part of the island, fantastic cliffs and dropoffs into the ocean.
Have a great time, we were there in 2007.