Always carry some spare change in case you need to go to the restroom.
…And a pay toilette is the only option. You’d be surprised how willing you are to part with a 50 cent piece when you need to relieve yourself (and how frustrating it is not to have one in the same case)!
I also read in the guide I have that for free public restrooms, there’s often a porter there who you are strongly encouraged to tip anywhere from 50 cents to 2 euros. It’s expensive to do your business in France!
