Cerro Negro is the headline reason most travellers stop near La Paz Centro: a steep, jet-black cinder cone rising abruptly out of the flat Maribios plain, and one of very few volcanoes in the world set up for tourist "volcano boarding". The hike to the rim takes about 45 minutes from the access road, and the loose ash slope on the back side lets you slide down in a few minutes flat — sitting on a reinforced plywood toboggan, kitted out in goggles and an orange jumpsuit, often hitting 50-60 km/h.
Operators are based in León, 25 km north, and run small group trips daily — Bigfoot Hostel runs the best-known one. Independent travellers can sometimes negotiate a ride and entry with a passing taxi from La Paz Centro for around 30 US dollars including the 100-córdoba park fee. Wear closed shoes and bring a bandana for the ash; you will be filthy by the end.