I have a little different suggestion: When you arrive at CDG you take a shuttle bus and check into Ibis at Roissy-Pole (there are 3 Ibis next to cdg, the one I am suggesting is right next to the train station) they have a per hour formula at 10euros/hour so a rest of 5 hours will cost 50 euros and will be in the early morning when Paris is the least pleasurable. After this rest, you will be able to enjoy your day much more and the access to the train will be right next to you saving you the time and hassle of another shuttle bus. You then buy a day ticket "Paris Visite" at 16 or 17 euros which will take you to Paris and back and will allow you unlimited access to buses/metros. After spending the day in Paris, you take the train back to CDG2 from Chatelet or Gare du Nord stations. But beware to take the train back a little early if you don't know your way around very well in Paris. Have a good day.