The drive up Haleakala is worth it all by itself. It's not quite on the scale of the Hana road, but still worth it. Depending on where you have been on Maui before, there are interesting things to see, like the giant cacti in the fields, the groves of eucalyptus trees, the panoramic views, etc. As you drive up, you pass through several very distinct zones of vegetation, starting with the sugar cane fields at the bottom, and ending up in the nearly barren volcanic rock up top.