I just wrote about the "free mobile offer" for people coming in France with an unlocked simphone. A new solution, easier to use for a short trip, has been launched by another network operator, Bouygues.
It's a prepaid sim card under the brand B&You that's a sub-something of Bouygues Telecom. You can get one fo 5 € and can charge it for voice or data. For example a 400 MB data will cost you (right now) 15 €.
The biggest issue, as with Free Mobile, is that you can only get it from their web site, so you have to understand french to order one (and have it delivered where you will be staying). There's no limit on how long your fill up can be used so you can keep it for next time you come and don't have to buy a new sim card each time.
They even have nano sims for iPhone 5.
Go to B&You