I had a 10 hour layover in CDG earlier this year and took the train into Etoile and ended up walking about 12 miles through Paris with my girlfriend and ended up at Gare du Nord which we found a train to go back to CDG. It was super worth it. Basically just walked towards the Eiffel Tower from Etoile (Champs Elysees) and then up the Seine, through the beautiful gardens and stopped by Notre Dame, then walked towards Sacre-Couer and then on to Gare du Nord.