León sells volcanoes. Cerro Negro is the famous one — you walk up a live cinder cone carrying a plywood board and slide back down it — and every agency in town sells the same trip at much the same price, so the difference is group size and whether the guide bothers with the geology.
The better walking is elsewhere in the Maribios chain: Telica, where you can look into an active crater and see the glow after dark, San Cristóbal, and the traverse. The city itself is walkable and free: the cathedral roof, the murals, and the Ortiz Gurdián art collection.