On the southeast side of the island, an amazing trek that explores the crater combines the Sliding Sands Trail and the Halemauu Trail to venture deep inside the crater. Its an 11 mile route that requires about 9 hours. The Sliding Sands Trail starts on the rim at 9800 feet and descends to the crater valley floor at 6600 feet. This is the larget volcanic crater in the world. Head back out of the crater via the Halemauu Trail. This is a one-way route that requires shuttling between trailheads which are miles apart and at different elevations.
To reach the trailhead from Wailuku or Kahului, head east on SR37 (Haleakala Highway) for about 11 miles to Pukalani. Turn left heading east on SR377 (also Haleakala Highway) for about 6 miles. Turn left heading east on SR378 (Crater Road) for 17 miles to the Haleakala Visitor Center trailhead.
Here are some notes on adventuring in Hawaii. First, unlike mainland forests, the tropical island forests can grow very dense and can mask dropoffs. Plus, the trails can be very wet and slick most of the year (December to February is the main rainy season, but expect rain all year round). Stay on the trail to avoid disorientation. Second, speaking of disorientation, be aware that most Hawaiian topographic maps use the Old Hawaiian Datum. When trying to use the latitude/longitude from those maps with a GPS unit, you will think you are 0.4 miles south/southeast of where you really are. Kind of important in a dense jungle. Third, the bacteria Leptospirosis is found in the local waters and mud. It causes flu like symptoms within 2-20 days of contact, and can be fatal if not treated with antibiotics. Fourth, when hiking about, walking sticks can be very helpful for stablity in the slick conditions, for probing mud hole depths and clearing webs of the tiny crab spider.
Overall, Sliding Sands is the 5th most popular hiking trail of all 45 hikes in Hawaii.