Cafes in France have restrooms for customers only. If you need to use the restroom in a café, the cheapest way to do that is go stand at the bar counter and order "un café" (an espresso shot.) Usually 2 or 3 euros. If you sit and need service, it costs more.
Bakeries are a great place to get lunch on the bike. You can leave your bike outside, go in, get a sandwich, a drink, and a pastry and you're out the door in a flash. Some bakeries have places to sit and don't provide service so you can sit and eat there. Or, find a park or church nearby.
A particular thing about France is that it is universally expected that people say "bonjour" before saying anything else. Everywhere and without fail. If you dont say "bonjour" before speaking or engaging, they might look at you with a weird look until you say "bonjour." Maybe a touch odd for us anglos but that's the way it is. Also important to say "merci" after interactions and when leaving; "merci. au revoir" Seems obvious but I'm always struck by how much I DON'T do this at home because it's not expected.