Nobody tells you how empty the middle of Hawaii is. The Saddle Road runs between two enormous volcanoes, Mauna Kea to the north and Mauna Loa to the south, across a high plateau of lava, scrub and cloud, and the access road to the Mauna Kea visitor station leaves it at a junction that is easy to miss. There is no town, no cafe, just a turning and a ridge of dark cinder cones. Everything on this island looks smaller than it should, because the mountains are so large that they do not look like mountains at all.
The road itself, officially the Daniel K. Inouye Highway, was rebuilt in the 2000s and is now a smooth, fast and rather lovely ride. Long, open curves cross old flows, with views of Mauna Loa's gentle dome and, on clear days, of the white observatory domes on the summit of Mauna Kea. At around 2,000 metres the air is thin and cool, and cloud rolls in and out in minutes, so do not be surprised by wet tarmac, dense fog, and drifting mist on an otherwise sunny day. Nene, the Hawaiian goose, wander near the road in places. There is also a military training area near here, so you may meet convoys.
The side road climbs to the Onizuka visitor information station at roughly 2,800 metres, where people acclimatise before the summit. Mauna Kea is sacred to Native Hawaiians, and the summit has been a site of long-running dispute over telescope construction, which is part of what you are looking at.
Do not run low on fuel in this part of the island. There are no stations along the Saddle Road itself. Waimea, about 37 kilometres to the north, is the closest town with a proper range of shops and petrol, and Hilo and Kona are the other ends. Rental agreements sometimes restrict the road, so check the paperwork. Bring a jacket, because it is far colder than the beaches you left behind.