Haleakala means "House of the Sun," and from central Maui it does appear as if the sun comes right out of the top of the mountain. A Hawaiian legend says that the used to go through the sky so fast that no one could get any work done, so the hero Maui (after whom the island was named) climbed to the top of the volcano, captured the sun with a magical net as it was starting to rise, and forced it to agree to slow down.
Haleakala Crater is a National Park and there are many hiking trails through the crater. According to a local story there is supposed to be a heiau inside one of the inner volcanic cones. There are tours to take you to the top to watch the sunrise (it's a powerful experience, and a cold one), and there are bicycle tours from the top all the way to the bottom.