This one is hard. France requires the SIM to be registered. You can buy one and do this in advance, by mail or fax; or do it there. Recommendation: do it at an Orange store. Don't buy a SIM, even an Orange SIM, anywhere else. And make sure they register you on the spot.
Here is my story, full of missteps.
We arrived late on Friday, too late to go to an Orange store (open 10-7 generally, sometimes 930-730). We did not see a mobile carrier booth in the airport, just a third party reseller, Relay, and skipped it.
The next morning we had a train trip to Caen to take a tour of Normandy, leaving before the stores open, and returning after they close. The next day was Sunday, when they are all closed. Stuck until Monday?
Before boarding the train, I went into a Relay shop - they are all over the place, like 7-11's - and bought a €10 SIM plus a €10 Data Pak for 500mb. Problem: the SIM worked right away, but couldn't be recharged with the Data Pak until registration. A mail package comes with the SIM to register by post. Only takes 8-10 days! Not an option for a tourist.
I found WiFi during the tour and went online to find an online way to register. I couldn't find a registration page in the Orange.fr site, but did find a traveler site that allowed online registration. It looks legit but then most Phishing sites do. Unfortunately, it doesn't really solve the time problem since the registration needs to be approved during normal working hours - Monday.
Sat and Sun were shot. Disconnected!
On Monday I took a train to Chartres, leaving before the Orange stores open in Paris, and found the Orange store in Chartres after I arrived. It is behind the McDonalds, up the hill from train station. (It is NOT where Apple maps says it is.) McD's has free WiFi which helped me find the Orange shop.
At the store I registered online at the counter, or so I thought, and went off to see the Cathedral (well worth it - take the free tour from the inimitable Malcolm Miller, an English docent who has been there since 1958 and says he will be there until the Second Coming).
This too didn't work. I still couldn't recharge the SIM, even after waiting until the end of business hours.
I went to another Orange store after I returned to Paris, and they finally got me connected. They said they couldn't make the fast online registration (which they handle for you) work off the Relay SIM, so I bought a new SIM for €10. They got me registered online, and did the 500mb recharge. This worked. Viva la France!