I’m not especially well-versed in United States National Parks (although I’m trying to catch up), but Haleakala Crater has to be one of the coolest parks in the country. Haleakala is a volcano that actually, unbeknownst to me before this trip, forms about 75% of the island of Maui (a fact that has since been confirmed by the luminous scholars at Wikipedia). One of the absolute essential excursions during a trip to Maui would be to visit the highest part of the volcano, from which a massive crater can be seen.
Most travelers to Maui take the Road to Hana on the northeast end of the island, from Kahului to Hana. You absolutely should do so, as well. However, many people completely ignore a similarly spectacular back road to Hana called the Piilani Highway, and miss out on a completely different landscape. I recommend that you avoid overlooking this alternate route.