If you are using a prepaid SIM card, make ABSOLUTELY sure you know EXACTLY how to reload it, and dont wait till that last minute. For one thing, French tabacs, which sell recharge cards, are closed on Monday (the one next to my hotel, anyway).
English language GPS is nice but when it mangles foreign street names, it can be infuriating.
Small portable tool are useful. This includes tweezers.
Always pack Pepto-Bismol (not available in france). Antacids, too.
Credit card companies will block access to your card - even a prepaid debit - if they suspect your 'pattern of purchases' indicates someone stole the card. You get to call them and list the last few, to prove its you.
Hats are a good idea, even if they DO make you look like a tourist.
Small smart phones are nice, but TOO small can drive you crazy.
Dropbox is a terrific service, although the upload function for Android could use a ´select all´ option.
The TGV is expensive and worth it.
The French autoroute tolls are automated and take euro coins. Some take euro notes. They dont take prepaid debit cards, even with chips. Chipped credit only.
French banks don´t like to give change. Stores don´t, either.
Internet access is not a given. Neither is air conditioning.