View looking into the crater atop Haleakala Volcano on the island of Maui.
You can drive to the summit of Haleakala
Haleakala Volcano is a national park. There is a visitor center not far from the peak, which reaches 10,023 feet (3,055 meters).
A road can be taken all the way up the volcano to the peak, and then there are trails for hiking down into the crater. The “crater” at the summit interestingly isn’t a real volcanic crater or a caldera, but was formed by erosion of the mountain over time.
Two hiking trails lead toward or away from the summit, depending on which direction you’re going. The high altitude can induce headaches or other manifestations of altitude sickness, so caution and preparation are very much needed. Additionally, the air near the summit is cool and can potentially drop below freezing any time of year.
For explanations of basic volcanology terms such as shield volcano, crater, and caldera, see Types of Volcanoes and Volcanology Terms.
The two volcanoes that comprise Maui are clearly identifiable in space photos. West Maui is extinct, although East Maui (also called Haleakala) is an active volcano that is expected to erupt again.  The smaller island to the southwest is another shiled volcano called Kaho'olawe. 
About Haleakala, also called East Maui
The Hawaiian island of Maui is shaped the way it is because it is formed by two shield volcanoes, called West Maui and East Maui.
West Maui is also known as Kahalawai, and it last erupted about 320,000 years ago and is thought to be extinct. East Maui is also called Haleakala, and its last eruption date is uncertain although occurred somewhere between the 17th and 18th centuries.
One theory is that Haleakala last erupted in 1790, based on a comparison of maps made by two different European explorers. Scientific dating methods however suggest that the date of the most recent eruption was 100 years prior to this or earlier.
The Big Island of Hawaii to the southeast of Maui is well known for its volcanoes, especially massive Mauna Loa and constantly-erupting (since 1983) Kilauea. However, the island of Maui has a volcano as well that is like some of the others comprising the Big Island – meaning it is not currently erupting, although is a definite threat for future eruptions.
Haleakala is considered active, a term that most commonly means that it has erupted recently in geological terms, in the past 10,000 years, and that it also remains a threat for future eruptions.
Haleakala forms more than three-quarters of the island of Maui, and the most recent eruption occurred along a rift zone which extends southwest from the summit. Another rift zone extends to the east, and runs past the town of Hana and under the surface of the ocean.
Most recently, Haleakala has erupted on average every few hundred years, and it is thought that it could erupt again sometime soon. However, there currently are no signs of this, and all remains calm and quiet – for now.