I know there are very few public WCs in Florence. Do they all accept coins only...or do some accept credit cards?
Bring coins. It’s generally 1 euro per entry to be inserted in the turnstile. Sometimes you just give the money to the cleaning lady. Credit card options are not available everywhere.
Restrooms at coffee bars are free for patrons, so you might have to buy just something small and inexpensive, like a glass of sparkling water or an espresso. Restrooms at museums are free, so take advantage of them when you are inside one.