The trip from Kona to the summit is a 2hour drive (4x4 or AWD needed from 9000'+), and well worth the trip. If you can time it to go with the caravan leaving from the visitor center that takes you inside the Kek observatory, the guides are very informative. Views of Maui in the distance are great, and otherwise you feel as though you on Mars, and definitely no place you've been before.
Mountain weather is always changing, but in Dec it was 50*F up top (though 34*F in the observatory during the 30 minute tour). Wear pants and shoes (the only time you'll wear them on the island is here), some bring water to drink, sunscreen, and you'll be prepared for the adventure
It's hard to believe they don't charge for the service; so much so that a nice donation in the visitor center feels like you are still getting more than your moneys worth. The next trip to the Big Island will include star gazing from the summit!
FYI kids under 16 aren't allowed on the tour