The easy 0.6-mile trail up the cinder cone Puʻu Huluhulu (Shaggy Hill; 6758ft) makes a piquant appetizer, and an acclimatization opportunity, before going up Mauna Kea. Inside a kipuka (volcanic oasis), the 20-minute hike climbs through protected native forest to the hilltop, from where there are panoramic views of Mauna Kea, Mauna Loa and Hualalai on clear days. Leave gates closed.
Puʻu Huluhulu erupted over 10,000 years ago in Mauna Kea's waning years. Some of its cinder was mined for road building in the 1940s but since the 1970s it has been protected, in large part simply by fencing it off from grazing pigs and sheep. Reforestation efforts have planted hundreds of mamane trees, an endemic from the pea family which is important food for the critically endangered palila bird (Loxioides bailleui). Look for this yellow-and-white breasted honeycreeper dining on the mamane's seeds which are highly poisonous to most other animals.
The trailhead parking area is almost opposite the turnoff to Mauna Kea, near Mile 28 on Saddle Road. Don't confuse this hike with the Puʻu Huluhulu Trail in Hawaiʻi Volcanoes National Park.