The Moucherotte summit is a great place to take photos from, looking down into Grenoble, or across at the Chartreuse massif. The simplest walk is a direct up and back (yellow path) from the car park (at 1240m), however there are a couple of alternative paths down. The red path is a nice alternative zig-zag through the forest (with a few eroded sections - care needed) directly back to the car park, and the blue route takes you over the summit, down Vallon des Forges and along Sentier de la vite? below le Trois Pucelles. I’ve taken the yellow and red paths, but haven’t attempted the blue one yet.
I think I’ve planned out a good path to get back from the bottom of the blue track to the car-park, however I couldn’t see a good route on the map. You may need to go via the town. Note that this route drops to 1100m, where as the car park is at 1240m, so there’s a short climb back to the car park when you get to the bottom.