Normandy by car

If Mont St Michel and surrounding area is your last stop in the region; I'd suggest returning the car at Rennes train station and take the train back to Paris from there. You'll arrive at Montparnasse station in Paris and it is a quick 10 minute taxi ride to your hotel.

Stick with the major brands-Hertz, Europcar, Avis, Sixt
See what AutoEurope.com offers
All drivers will need an IDP-required by law
Get at your local AAA before leaving home

ETA- Drivers with a US license are required by law to have an IDP.

All drivers will need an IDP-required by law

Really?
Absolutely not! Driving licences issued in the UK, the EU and EEA countries are accepted (no IDP required)

KD's suggestion to return the car in Rennes is a wise one. Also, picking up the car in Rouen (after spending the night in Rouen, I hope) is also a good idea.

Alas, I don't know how easy or quick it is to go from Gare du Nord to Gare St. Lazare, and I see no one has answered that question for you. Checking here on the forum with a search I found that posters say it is about a 2.5 km walk (35-40 minutes), or bus #26 between the stations, or a taxi is possible. I personally would take a taxi if I had bags, but I am old and slow. I also think the sidewalks could be slow traveling, if there are lots of people.
Re the IDP, if you are from the US it is required. Canada, if your license is both French and English (as in Quebec) it is not required. The UK and EU have separate agreements with France.
We always research car rentals at Autoeurope.com. They will have info and rates on several car rental companies. You can reserve through them or directly with the company you decide on. Sometimes Autoeurope is cheaper, sometimes the company itself. We have rented from various companies over the years and have had good luck with all of them. (I don't recall using Hertz.)
Be aware that September is about the most expensive month for Paris, but the tourist crowds will be smaller than in July and August. There will be lots of conventioneers, however.
Have a wonderful trip!

Nord to Saint Lazare is just one stop on the RER E train (from Magenta to Haussmann-Saint Lazare): if you have manageable luggage it would have my preference, as Paris buses can be quite slow and unreliable these days.

Tickets can be arranged on smartphones, or bought at the Eurostar onboard bar.

We're doing a very similar trip in October, except we arrive at CDG from Australia.

We were going to taxi to St. Lazare, then train to Rouen, then taxi to our hotel, but have now changed that to having a private driver take us from CDG directly to Rouen. We figure we'll be exhaused after 24 hours in awful economy, and therefore prone to confusion and pickpockets.

We normally always use public transport in France, but not just after arrival on a long flight.

We are then picking up a hire car from Sixt after three nights in Rouen and will keep that for our eleven night Normandy trip. We are returning the car to Caen after MSM, and then training to Paris, and will spend another couple of weeks using trains in France.

Since you go a month before us, I hope you write a trip report, I'll keep a watch for it.

Have a great trip.

Just to add on that Rennes is a great location for pick up or drop off for Western Normandy—especially Mont Saint-Michel. I have done both there. Unlike Rouen, there is a TGV from Rennes to Paris. Also, if it were me, I would either take a taxi between train stations in Paris if I have luggage, or walk/bus if i have no or minimal luggage. Personally, I find those modes are far preferable to the Metro. You actually get to see the city rather than tunneling underground (this may be less important for someone who lives in Paris and sees it everyday). I haven't found the buses unreliable in Paris at all, although they can be slower depending on time of day and traffic. But often, they are right outside of the station with no stairs, escalators, etc.

Really?
Absolutely not! Driving licences issued in the UK, the EU and EEA countries are accepted (no IDP required)

I know. People harp on about this incessantly (and usually incorrectly). Its like they get a commission from the IDP people, or something.

Canada, if your license is both French and English (as in Quebec) it is not required.

Actually Quebec drivers licenses are ONLY in French (and yet, miraculously, they manage to drive all through English speaking countries without requiring an IDP). New Brunswick, Ontario, Manitoba, Nova Scotia, PEI and Newfoundland all have bilingual licenses.

...its like they get a commission from the IDP people, or something.

Touché!

We did something like this last year. I looked into renting a car in Paris and decided to avoid all the hassle and annoyance and potential issues driving around Paris.

We took the train from Paris to Caen, rented the car in Caen, spent a few days in Normandy, returned the car to Caen, and took the train back to Paris.

You're on vacation. Avoid the Paris traffic and ride the comfortable French trains.

Thank you all so much ! Such good info. We have carry on bags, but will prob taxi to St. Lazare. I will search the car companies mentioned, should be no problem. Already obtained the IDP online; hope we never need it!.

On the way back from MSM; is it too out of the way to drive to Chartres? But then we are almost all the way back .. so then where would be a place to drop the car?

Really? Absolutely not! Driving licences issued in the UK, the EU and
EEA countries are accepted (no IDP required)

I know. People harp on about this incessantly (and usually
incorrectly). Its like they get a commission from the IDP people, or
something.

Oh dear I sincerely apologize to all I have offended
I assumed the OP was driving on a US license
I have edited my post with the clarification

I don’t understand how others are so sure that OP is not a US citizen with a US driver’s license?
Are only UK/EU citizens allowed to travel on Eurostar?

I do try to provide accurate information to this forum and always mention the IDP- assuming it is a US driver- I have seen travelers turned away at the car rental desk because they do not have it. Would hate for that to happen to anyone.

I see the OP has returned to let us know that they do have an IDP so now I must assume that OP is indeed a US driver.

You said "all" drivers need an IDP. The other poster and I were simply correcting your misinformation.

Perhaps better not to assume everyone is an American. Isn't that the Rick Steves way?

not only US drivers need an IDP or authorized translation from a panel of approved translators. Residents in most countries of the world - barring NB - do. A few don't.

I assumed the OP was driving on a US license

You assumed incorrecty. It seems to be a default assumption by American posters that unless a poster otherwise lists their home country under their user name, then they must be American. . However it has been amply demonstrated on these forums that there are many posters, some of them frequent posters, who come from all over the world. Perhaps it's time to make all posters locations mandatory instead of optional.

You said "all" drivers need an IDP.

I meant "all drivers in your party" as the post was directed only to the OP. Next time I will try to make that clearer.

You assumed incorrecty.

Actually no I don't think I did- as the OP has posted above:

Already obtained the IDP online

Could that mean OP is from somewhere else in the world that requires an IDP? I suppose it could.
Didn't those that were so offended by my post all assume that the poster was NOT American?

not only US drivers need an IDP or authorized translation from a panel
of approved translators. Residents in most countries of the world -
barring NB - do.

Thank you Nigel

Anyway I agree that it would be nice if all members included their location. That would avoid so much distress.

I am from NY, I should have clarified the US license. I prefer no potential problem at the rental desk, and my French is rusty at best ! Any thoughts on the drive back to go thru Chartres.. vs. train back from Rennes ?
Thx

It's about 4 hours to Chartres via Alençon, a bit less (but much more boring) if you take the highway via Le Mans. You'd get a half-day in Chartres, unless you stay the night. Feasible, but possibly rushed!