The Causses and the Cévennes cover a really large region, and as others have noticed it is not immediately clear where to go to see its essence (it's a cultural landscape). Driving up north from Toulouse (which takes over 2 hours), I decided to take the highway from Lozère to Millau and take some detours from there. This highway already crosses the core zone by the way.
My first goal was to see the Cirque de Navacelles. These "Cirques" are one of the peculiar features of the landscape here. They are amphitheater-like valleys. Standing on the edge, it looks just like an enormous meteorite has landed. But it was all made by glacier erosion. This site lies about 28km from the main road. I also saw lots of large birds of prey (vultures?) circling around there.
It was back to the main highway until I reached Millau. A small mountain road from there reaches further inland and finally ends at the Chaos of Montpellier-le-Vieux. This is a karst landscape with strangely shaped rocks, just like the Chinese love them. In the summertime, it is a very popular tourist site, but I only encountered one other couple in the parking lot. I choose to do the 1.5-hour red hike around the park: a pleasure to just be outside in nature in this fine weather. At the end of that walk, I finally encountered two man-made constructions in the landscape that are pointed out in the description of this WHS: a cistern and a cave that has been used for shelter. Both were used by roaming shepherds.
In hindsight, I could have done with some more time on the ground here. I managed to visit the Millau viaduct also but did not have enough time for a tour through the sheep's cheese factory at Rocquefort. I would be interested to hear from someone who did make that trip.