Hi Deb,
If I were arriving and leaving by CDG Airport, I would recommend hiring a rental car from CDG Airport itself (which is about 35 minutes North East of Paris). The rental car staff at CDG all speak solid English and if one can avoid driving within the city limits of Paris, I would
highly recommend it. Paris is amazingly confusing in terms of streets and directions. Even the locals swear by GPS's. Parking is nearly impossible to find. Plus, local drivers and especially the motorcyclists/scooters are very aggressive. New York City is perhaps similar in terms of traffic/driving style, but there's a million more motorbikes zipping in between lanes here.
If I were forced to give a recommendation of which Paris train station from which to rent a car, I would say either
Gare Montparnasse or
Gare de Lyon, which are in the southern and south-eastern areas of central Paris, a little bit farther from the hectic North and North West stations of Gare St. Lazare,
Gare du Nord,
Gare de l'Est.
I hope this helps. Do let me know if you have other questions.
Best regards,
Ben