My flight arrives at 08:30 to Charles De Gaulles Airport and I fly from there again at 18:40 (same day). Do I have time to sight seeing Paris, maybe have a quick lunch and return back to airport for my next flight ? What is the best transport mean from and back to airport ?
RER B train travel time from CDG to Paris Gare du Nord / Chatelet Les Halles is about 35-40 minutes. You should have enough time for a quick visit.
Buying a Ticket Mobilis 1-5 zone day ticket is cheaper than two one-way single-use tickets and you can use it an unlimited number of times until midnight, good for any Paris Metro, Bus, Tram, RER train.