The magnum opus of Mother Nature in Réunion, Piton de la Fournaise is the island's most famous natural attraction. Simply dubbed le volcan (the volcano) by Réunionnais, Piton de la Fournaise is not a dormant monster, but an active geological wonder that erupts with great regularity; in April 2007 the central, 900m-wide crater collapsed by 300m, and new lava fields were formed on its southeastern flanks, down to the coast. In August 2015 a new eruption occurred, though less powerful.
The good news is that it's one of the world's most accessible active volcanoes, and it's possible to hike up to the crater rim. You can also fly over the volcano or simply enjoy the scenery from a viewpoint right on the crater's outer rim, where the road ends.