Note: As of mid-2019, access to the summit area may be blocked by protesters and/or police. Before attempting to drive or hike to the summit, get the latest info about this situation.
Mauna Kea is often called the highest mountain on earth, if you measure from its submarine base. This is a difficult claim to back up with objective data, but I do believe that the angle of the slope that descends 30,000 vertical feet from Mauna Kea's summit is greater than that of any other vertical drop of that magnitude on earth.
The summit area of Mauna Kea is a dry, windswept collection of little lava rock hills, utterly devoid of vegetation. Perhaps the greatest concentration of astronomical observatories in the world is located in the summit area, all taking advantage of the dry climate, high elevation, and moderate slopes that allow a good auto road to ascend all the way to the summit. The start of the road is dirt, but the top section has been paved to minimize dust that would harm the delicate telescopes.
Most people drive cars to the summit. Most rental cars on the Big Island can not be legally driven on the Saddle Road or the Mauna Kea Road, but many tourists ignore this restriction and have no problems. Or, to be safe, you can rent a SUV for more money that is allowed up the peak. The main issue is the high altitude--sensitive individuals could have severe altitude sickness with the rapid rise from sea level.
There is a trail up Mauna Kea that climbs from the Visitor Center near the Saddle Road for those who wish to hike through the volcanic moonscape. A final option (available in some winters) is to ski Mauna Kea, using cars from a local outfitter as your ski lift.