Haleakala is a large shield volcano that makes up the majority of the island of Maui. The park is divided into two districts, the Summit District and the coastal Kipahulu District. I recommend starting your visit in the Summit District at the Headquarters Visitor Center to get your bearings, although there is another Visitor Center at the peak.
Upon entering the park, you begin to get a sense of the awesome landscape that is so full of rich colors and tumultuous history. The drive up to the peak is a steep ascent. Once at the top, enjoy the crater vistas. The true summit is Pu'u'ula'ula at 10,023 feet. Then, hike into the crater. The popular 2-mile moderate hike brings you down the sloped side of the cinder. I think the best views of the park occur here. Just remember that it takes twice as long to hike back up, as weather changes quickly here.
Watching the sunrise from Haleakala is very popular despite the early hour (sunrise hours are from 3-7AM). Reservations must be made in advance. Otherwise, you will not be allowed into the park until 7AM. Watching sunset does not require a reservation.
On your drive back down, stop at Leleiwi Outlook. There is a short .3-mil round trip hike to an overlook that provides great views of the crate and Ko'olau Gap. Further down the road is the Halemau'u trailhead, which is a 2.2-mile round trip rocky path that features a natural land bridge and culimates with a stellar crater view. Out past the park entrance is Hosmer Nature Trail, a short walk that takes you by some rare plants.
At the remote Kipahulu District, a much different experience awaits. You'll find ocean views and towering waterfalls. The Pipiwai Trail is a 4-mile round trip hike that takes you to two waterfalls and through a bamboo forest. A short trail departs near the Kipahulu Visitor Center. This .5-mile round trip goes to a Hawaiian cultural demonstration area and archaeological sites. It ends at a collection of pools and an unbeatable ocean view.
Both districts offer camping. There are also three cabins for use with advanced resevations. These are accessible only by hiking, and are 3.7, 5.5, and 9.3 mile hikes.