My wife and I have used Auto Europe for our car rentals in France and Germany. Easily available at most airports with pickup and drop off places in the various cities. Rates are comparable to US. Most cars have manual transmissions unless you specify automatic - and are will into pay extra. Do not attempt to drive anywhere in Paris. It is not worth the stress and potential for accidents from crazy drivers.. Take a train to a nearby major city - or even the airports and rent from there. While gas is relatively expensive, with the two cars I rented, I was able to go about 900 kilometers (about 560 miles) on a tank of gas.
If you have time, get off the main highways from time to time and take in the scenic and bucolic countryside.
If not, take the train, leave the driving to them, and enjoy the scenery from your seats windows. (Never think of AMTRAK - no comparison). Break up your journey with a stopover or two on the way. Or you can pretend that you are still in your early twenties and use the train as your hotel a la Eurailpass.
I'm flying EasyJet from Paris to Rome - mainly because I need to get there quickly. I'll report back after my trip.
Enjoy your trip.
