Haleakala National Park stretches from the south coast near Hana (at the Pools of Oheo) to the 10,000-foot Red Hill Summit of the still-active volcano—nearly 100 miles of trails. If you can only do one—which would be a shame—try the Sliding Sands Trail, which drops from the visitors center near the summit ato connect with a spider web of trails in the vast red-hued crater. (Okay, Haleakala's central depression is not technically a crater, but an eroded valley that is dotted with old cinder cones, or pu'us, as they are called in Hawaii.)
When heading down the Sliding Sands Trail, bear in mind that you will have to hike back. It's easy to be lured farther to overlooks and cinder cones, and then be left with a long trek back up, made more difficult by the elevation. Also be sure to bring plenty of water, emergency food, and clothing that will protect against extreme cold and heat, which may occur on the same day. And don't forget to make sure the camera is charged and the chip is empty, since photographers will get a happy trigger finger.