From the Crater Rim drive, head south on the Chain of Craters Rd for ~3mi, and turn off left into the signed Mauna Ulu trailhead.
(Route description as it existed in February 2004. Given the constant activity in Hawaii Volcanoes National Park this trail may easily be subject to change or closure -- best to check in with the Park Service before planning to hike it.)
From the trailhead, follow the initial trail towards Pu'u Huluhulu -- this is a small hill located approx 1mi away from the trailhead where a number of tourists hike at night -- on a clear night sometimes the glow of the lava from the Pu'u O'o vent far to the east can be seen. The trail through this first section has reflective markers along the walk to keep visitors from getting lost at night.
At the junction ~1mi from the parking area, bear right onto the continuation of the Napau Trail as it winds out into the Mauna Ulu lava shield. The trail here becomes less distinct and you'll follow from cairn to cairn through the desolation. Hiking along the old lava flows is rough and there's no shade at all. The trail climbs briefly bypassing a crater near the summit of Mauna Ulu, then descends again through numerous steam vents and skirts the rim of the Makaopuhi Crater before diving into the rainforest.
After ~5mi from the trailhead, you'll reach a junction with the Naulu Trail -- continue on straight - passing through areas of rainforest, old lava flows, steam vents, the remnants of an old Pulu factory, and meadow-like areas before arriving at another junction just before the Napau Crater (~7mi from the trailhead). Continue straight ahead for a good viewpoint from the west rim of the crater and your first views of the Pu'u O'o vent. Back at the junction, take the side trail north, and you'll pass through the Napau backcountry campsite area; several good sites for backpackers here. Passing a warning sign by the park service concerning the potential hazards ahead, the trail continues on, passing around the north side and descending into the Napau Crater. The terrain becomes a lot more desert-like in passing through this area with gnarled rock formations, cinders, more steam vents, and signs of more recent lava flows. After another 2mi or so (~9mi from the trailhead), you'll reach the end of the trail at the base of the Pu'u O'o vent (still a couple of miles from the active vent) with a go-no-further sign. In February 2004 there was an active lava flow in the area here -- fascinating to see up (fairly) close.
Trail (~18mi roundtrip, gain ~1000' elevation)
Good hiking boots, sunscreen