Get the Visit Paris travelcard .... see

http://www.ratp.info/touristes/index.php?l...ge=presentation

for details and

http://www.ratp.info/touristes/index.php?l...ge=paris-visite

to see where to purchase the card

If you get one at CDG ( zones 1-6 ) then the trip from CDG to the city centre of Paris et v.v. is included which is normally EUR 8 each way which makes the card even better value ( depends how long you are there ).