While visiting the Big Island, you might want to take a break from sun and surf to climb the tallest mountain on earth: Mauna Kea rises "only" 13,796 feet from sea level, but if measured from its base on the ocean floor the dormant volcano is well over 40,000 feet. Unlike Everest, which requires a major expedition, the Famous Summit of All the Land (as the Hawaiians called it) can be achieved with a round-trip hike of a half-mile. Bring warm clothing, it's very cold up top.
Still, some planning is required. Passenger cars can drive to the Onizuka Center for International Astronomy, which sits at 9,000 feet, but a four-wheel drive vehicle is highly recommended (sometimes required) for the last 8 miles to the trailhead. A number of tour companies take vans to visit the dozen or so astronomy observatories at the top—but if selecting this option, make sure your company offers the peak hike. If driving up, you should stop at the Onizuka visitors center for about an hour to adjust elevation and drink fluids to avoid altitude sickness--headaches and dizziness that can occur after climbing so high over a short period of time. (The symptoms are alleviated by descending to lower elevations.)
See Hawaii the Big Island Trailblazer, beginning page 180, for more tips and details for taking this spectacular mini-adventure.