Hi Ben
I went to Hawaii in 2012 for the Venus Transit and went up MK twice. First time to observe the transit at the summit and second time with an organised tour (Mauna Kea Summit Adventures see
http://www.maunakea.com/ )
While you can take cars up to the Visitor Info Station (VIS) at 9000ft, past that a 4WD is required and the only way is via a tour or on Saturdays you can join a convoy that goes up from the VIS. You must have a 4WD (not AWD) to join the convoy!
The VIS also holds regular viewing nights
The tour I did was excellent with MKSA. They picked up from near the hotel, had a meal at the VIS then up to the summit and wandered around the various telescopes and sight while waiting for sunset. the sunset is spectacular, one of the best things I have ever done! then back down to an area near the VIS to set up a coupe of C11 scopes for a couple of hours stargazing. That can be a little less interesting for southerners like us, as many of the people on the tour were observers and they all want to look at Omega Centauri, 47Tuc etc that they cannot see in the US, while I wanted M51 etc!! Having said that, I am heading back to Hawaii in a month with the SWMBO and I am dragging her up the mountain on the same tour.
Be careful of the altitude, it is not bad, but you find you just have to be a little more careful and teh brain gets a tad fuzzy at 14000ft!!
Where are you staying on the Big Island by the way?
Malcolm