Mauna Kea, on the Big Island, is Hawai'i's highest mountain at 13796 ft (4198 m). Rather than a single peak, Mauna Kea is undefined; it is a series of volcanic cinder cones, some red, some black, pasted onto a gargantuan massif. While it is tempting to draw grand analogies about how Mauna Kea embodies the spirit of Hawai'i itself, the real king mountain of the Big Island is Mauna Loa, which regularly spreads lava flows over a radius of 50 miles in almost every direction. However, because Mauna Kea's slopes are steeper than Mauna Loa's, the former's summit views are better, and its hikes shorter.
The maintained Mauna Kea Trail is 6 miles in length (one way) and climbs 4576 ft (1395 m), starting from the Onizuka Visitor Center, which is at 9200 ft (2804 m). The trail loosely parallels a partially paved summit road, and from the Mauna Kea Ice Age Reserve boundary at 13200 feet to the summit road's high point of 13700 feet, actually follows the road. Getting to the summit from road's end requires a mere 5-15 minute walk and 100 feet of elevation gan. The vast majority of "mountaineers" drive to road's end, which can be kind of discouraging for hikers, especially when the altitude starts kicking in. Aside from the moral superiority you can enjoy from hiking, you'll take in splendid and wild views along the way.
An interesting side trip is the short jaunt to Lake Waiau, a very shallow lake (no more than 10 ft deep) about a mile and a half from the summit. You aren't supposed to disturb the lake, and you probably wouldn't want to since Hawaiians used to bring the umbilical cords of their babies here and place them in the lake to give them "the strength of the mountain". If you must drink from the lake, however, today's modern filtration systems should take care of any lingering placental matter.
From either Hilo or the Kona/Waimea area, access the Onizuka Visitor Center via Saddle Road (Highway 200). The Visitor Center is 6 miles upslope on Summit Road, the turnoff to which is near the 28-mile marker off of Saddle Road. Warning! As of March, 2003, there are no signs marking the turn to summit road. Apparently, the signs keep getting stolen. The ranger speculates that it's astronomy students from UH-Hilo, but it's anyone's guess.
The trailhead for the Mauna Kea Trail is a hundred yards northwest of Onizuka Visitor Center.
No permits or fees. Rangers do ask that you fill out a Visitor Information Sheet and drop it in a box next to the phone at the Visitor Center, but they will only use this information in case of an emergency.
No camping in the area. Acclimatizing is very important since you will more than likely be driving up to the mountain from sea level, or, even below sea-level if you have spending your sun-drenched days scuba-diving. So consider camping at the Mauna Kea State Recreation Area, 7 miles west of Summit Road (the road that goes to Mauna Kea), at about the 35-mile marker on Saddle Road. Try to go during the week, as hunters (of pigs, goats and birds) both local and of the tourist variety usually have dibs on the weekends. If you can't camp, hang out at the Onizuka Visitor Center before making the trek up. You can learn everything you wanted to know, and more, about the different observatories on the summit.