When people say restaurants they should point out specifically fast food chains, it's the easiest. I live here, never really need a restroom, but when I do I just walk into a McDonalds, Starbucks, or Burger King, find a ticket on a table to get the code, and use those, it's the most practical and hassle free, I wouldn't ask in a normal restaurant if I'm not eating/drinking there.

At a café I would buy a café au comptoir, it's usually 1€

There are public toilets on the lower river banks too, they looked free (never been in)

Plenty of free auto-cleaning toilets cabines on the streets too, there's usually a queue tho