You can do this as a loop if you don't mind a short walk back on the road. Otherwise, choose one of the two trailheads shown for an out and back.
Perilous: sharp lava, uneven footing, and slippery rocks down by the blowhole.
The north parking (which you'll come to first if coming from Lahaina, Kapalua, etc.) has a longer and more interesting approach. Start down an old jeep road, then curve back towards the highway for a short ways. Look for the "lighthouse" (really a reflector station) and head in that direction. You'll come across a nice grass plateau where someone has lovingly created a labyrinth.
Continue on sharp lava from here with spectacular views of the rugged coast. Work right when you get close to the water (don't get too close - incredibly dangerous here!). You'll find a small blowhole, but continue working right until you get to the big one in about a quarter mile by following the lava benches.
The actual blowhole is incredible - a two-foot hole on a flat patch lava. It looks like a drain until water spurts out up to 40' in the air! Do NOT get too close or try to look in. Rumors hold that people have been sucked in and died.
Note all the cool etched lava that looks like honeycomb. Millennia of salt spray and bacteria have created these patterns.
Head your way back up the hill. This latter part of the trail goes pretty much straight up a gully back to the highway and the second (southern) parking zone.