We've heard the toilettes on the streets are of high quality. However, if this puts you off, there are plenty of other locations you can go. Creative ideas include:
-Public locations, such as town halls, libraries, municipal courts even.
-Educational locations such as Language Schools (France Langue, par exemple), universities, institutes, etc.
-Embassy buildings.
-Churches.
-Touristy buildings. Museums are especially great.
-McDonald's, or other fast food restau. They don't expect you to buy.
I have had the need whilst walking and dipped into the local language school. So perhaps do a little research and you're golden!
FYI: using a bathroom in a cafe and leaving without sipping coffee consitutes rude behavior. These tips can help avoid that.

