West of the Piton de La Fournaise, one of the most active volcanoes in the world, is the Plaine des Sables, a vast desert area made of solidified lava and lunar landscapes. As anywhere near the volcano, no vegetation is obstructing the hike except some "branles verts" (Erica reunionensis) around the Piton Rouge. I wouldn't advise anyone to do that hike when the fog comes up. It's not fun to spend the night with near freezing temperatures and no shelter. Otherwise it's a nice hike with nice views on the two highest points of the island, the Piton des Neiges and the Piton de La Fournaise.