There be Views!!
To reach this state high point you have options.
Hop on a local tour and be driven to the Observatory area, and walk the short uphill to the top...
Abuse your rental car and drive up the washboarded road to that self same area, and walk to the top...
Drive up to the 11,000' or 12,700' points on the peak road, park at the parking lots... and hike over and up the rough trail to the top.
Hike from the Visitor Center at the 9,200' level... reaching the top the hard way along the hiking trail.
However you choose to visit, plan on the effects of very high altitude (13,796') and potentially violent weather. Even though you will be as far south as you can get in the USA, the altitude creates conditions closer to those found in Alaska! Go prepared.
The road is listed as 4WD... mostly for the downhill. Standard cars may not have the lower gears needed to avoid abuse of the brakes. You could violate the terms of your rental car agreement if you get into problems up there.
Some of the maps still list the Saddle Road as a potential violation of the rental agreements, and that is the paved road to the turnoff for the Visitor Center! Really, it is now much improved, when completed it will be smooth, straight and enjoy wide shoulders. The end away from Hilo still enjoys the twisting, curving, narrow, no-shouldered aspects that make for dangerous driving and hazardous biking.
Check in with the Rangers at the Visitor Center for current road conditions.
If you do drive to the top, consider parking at one of the upper lots and hike down and over to the lake at 13,000'. It will give you a feel for both the terrain and high altitude walking.
Mauna Kea: White Mountain... classified as 6th highest state high point.
Well Worth the visit.
Check out the Triplog.