An alternative to scaling Stromboli's summit is the hour-long climb to this viewpoint (400m, no guide required), which directly overlooks the Sciara del Fuoco (the blackened laval scar running down Stromboli's northern flank) and offers fabulous if more distant views of the crater's explosions. Bring plenty of water, and a torch if walking at night. The trail (initially a switchbacking road) starts in Piscità, 2km west of Stromboli's port; halfway up, you can stop for pizza at L'Osservatorio.
During active periods, explosions occur every 20 minutes or so and are preceded by a loud belly-roar as gases force hot magma into the air. After particularly strong eruptions, you can watch as red-hot rocks tumble down the seemingly endless slope, creating visible splashes as they plop into the sea. For best viewing, come on a still night, when the livid red Sciara and exploding cone are dramatically visible.
Arriving here around sunset will allow you to hike one direction in daylight, then stop for dinner and more volcano-gawking at L'Osservatorio on the way back down. Making the trek just before dawn is also a memorable experience, as you'll likely have the whole mountain to yourself. From Piscità it takes about 30 minutes to get to L'Osservatorio, and another half hour to reach the viewpoint. Be prepared – the climb gets steep towards the end.