Disclaimer: I am French, I have used autoroutes in about all parts of France :)
TL;DR: There's normally no planning needed, the aires are sufficiently close together that you can start searching when you need one and get to a suitable one within 10 min/20 min assuming you're not caught in traffic jams (a real possibility during school holidays, look-up predictions beforehand if you travel Friday-Sunday).
The French "autoroutes" have two types of rest areas (though I had never paid attention to naming them, so I'll reuse @Relaxed names):
- "aire de repos" are rest areas with parking space, generally some picnic tables, and a toilet block
- "aire de service" are rest areas with parking space, generally some picnic tables, a petrol station and the station building also generally hosts a little shop (with a toilet inside) and even sometimes multiple shops and a "restaurant"
Both categories of "aire" alternate, and it is entirely feasible to only use "aire de service" which are much more comfortable (and correspondingly generally more crowded).
I am not particular myself, but do be aware that toilet blocks are generally:
- not heated
- not too clean
- and therefore smelly
- may not have toilet paper or anything to dry your hands (after washing them with cold water)
Being less "friendly" though, they are generally not as crowded, so if time is of the essence they will allow a more speedy "pit stop". It depends which level of comfort you wish for.
On the other hand, toilets in shops are heated, generally quite clean (as it reflects on the shop's image) and generally have toilet paper and something to dry your hands with (after using warm/hot water). They may slightly less clean or not have TP during rush hours.
The shops that come with gas pumps also offer a typical "mini-market" assortment of canned drinks, coffee machines, prepared food (sandwiches, salads, cookies, sweets) as well as various utilities at the very least. They may have, as mentioned, a restaurant for hot meals.
Since a couple years now, some of the supermarket brands have acquired a few of the aires (E. Leclerc, Carrefour, Auchan, ...) whereas only petrol brands had stations before (Total, Esso, ...). The supermarket ones are supposed to offer cheaper gas and services, but I've not found the difference that impressive to be honest: the prices remain much higher that what you'll find outside the autoroute anyway.
So, how do you pick where to stop?
This is a typical sign (from Wikimedia: Autoroute A62 panneau aire des Landes):
Below the name and distance, you can find a collection of pictograms.
First row, left to right:
- gas available (and even GPL: it's written underneath), it's 24/7 by default with credit/debit card
- hot drink available
- ATM available
Second row, left to right:
- picnic tables available
- collection area for camping car/caravans dirty water
You can find other pictograms on Wikia, I'll just borrow their restaurant sign for here since it's quite useful too: