I have done the drive up to Mauna Kea several times (and almost made it to the top of Mauna Loa but weather got in the way.) It is a stunning drive and completely amazing to think that you're probably the highest person on the entire Pacific Ocean. If you decide to do this you'll need to consider whether you plan to do any diving before/after, and whether you have any altitude issues. I stay at the visitor's center for about 40 minutes or so, then make my way up slowly. I rented a 4x4 from a place in Hilo that used to advertise itself as the only place that rented cars permitted on the Saddle Road. I'm not sure that's true anymore since much (all?) of it was repaved and improved. PM me if you would like details but there is so much local advice here already that you seem good to go.