As mentioned about the Saddle Road, make sure you have a full tank of gas. Rental cars are fine for the visitor centre but 4WD beyond there if you want to get to the summit. A couple of points to remember: Visitor centre is at 9,200 feet. The air is noticably thinner. If there is someone in your group or yourself with any form of respiratory issues, reconsider the trip. The summit is over 13,000 feet! Normal aspirated engines like your rental will be gasping for air when you get to the centre; at least our Impala sure was. And going down: use low gear; slow and steady.
If you own or manage Mauna Kea Summit, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.