Just did 14 days in France which started with a train from Paris to Bayeux where we had a lovely B&B five minutes from the town center. Owner picked us up at the train station and took us to the Hertz office. Stayed there three nights and then one in Honfleuer and another in Rouen, only because we were taking the TGV to Avignon the next day. We rented a car from Hertz in Bayeux and turned it in with a small drop charge at the train station in Rouen. Contrary to what is published on the Hertz web site the Hertz office which is in a local petrol station IS open on Sundays. If you're a big D-Day buff you will need a couple of days to do it right, otherwise Omaha Beach and the American Cemetery there will suffice. I had been there in 2004 and was surprised to find a very comprehensive welcome center (mini-museum) at the entrance. We also managed a quick side trip to Giverny to see Monet's home and garden. There you will find hanging in the home many Monet paintings that have never been in a museum. It was well worth the few hour detour from Honfleur to Rouen. If you can go to Honfleur you must go inside their church, the ceiling of which looks like two boats turned upside down. The Bayeux Tapestry is a must see and you should take the time to go through the museum section that explains the process used to create it. The Bayeux cathedral is larger than Notre Dame and equally as beautiful, and around to the left as you face the alter you can go beneath it to see the old burial area. Fascinating frescos painted there.
We did not go to Paris this trip except to arrive and depart, but we had been there before. Given the amount of time you have, assuming you have not been to Paris, I would suggest that you either split your time between either Brittany OR Normandy and Paris, or spend most of your time in the country and maybe three full days in Paris. Make a list of the highlights you want to see....Notre Dame, Sacre Coeur and Mont Marte, the Arc de Triomphe, the Eiffel Tower to name a few. I promise you will fall in love with the city and return to spend a significant time there in the future. One easy way to gat a quick feel of Paris, as well as the other big cities in Europe, is to get a 24 hour Hop On-Hop Off Bus Tour pass. They circle through the city and stop at all the big sites where you can get off and back on as you choose and stay as long as you like. Busses run every 15 minutes or so and if you do the full circle on the first pass it will help you pick out where you want to get off and see more, or not. Lastly, remember that getting the most from a trip like yours is about the quality of what you see and do and not the quantity. Enjoy.