Bathrooms

Best to clarify what you mean by "accessible". Do you need to avoid stairs?

WC is the general sign.

You should be able to find them in fast food restaurants and equivalents. For North American tastes French toilets may be a wee bit unisex.

I never had any problem finding toilets in Paris. They are readily available in museums, restaurants, train stations, etc. If by accessible you mean handicap accessible, I can't answer that. Do be prepared to pay for the public facilities (1€ or less, usually 50 cents), so keep some small coins with you.

Fast food restaurant chains will have public restrooms. You generally need to buy something, but a drink will suffice (ironically).

Museums are perfect for restroom breaks. Often you don't need to pay to enter the museum, they will have some in the lobby. Some major hotel lobbies will have public restrooms. Department stores are another good option.

Public pay toilets are found about Paris. You pay a small amount to do your business to the attendant (paid in theory to keep them clean and supplied). There are also pay toilet shelters that are port-o-potties on steroids.

Just about any cafe or restaurant will have restrooms for customers. If sitting outside, just go in and the staff will direct you where to go.

My rule is always use the restroom when one is available. But some people do have more frequent needs.

And don't forget that they are not called "rest rooms" in France or anywhere else in Europe.
WC (for water closet) or toilet--those are the words you'll need.

or the Pissoir (for men) in some areas. I consider myself fairly well traveled, but even I had a stupid grin on my face when I had to tell the lady attendant when she asked if I needed the Pissoir or the Toilette (because they were different prices)...

"There are also pay toilet shelters that are port-o-potties on steroids."

These are called sanisettes and are usually free. On some city maps, they're indicated. Cafe toilettes are often down a narrow steep stairway and thus difficult for some to use.

http://europeforvisitors.com/paris/articles/paris-public-toilets.htm

The "sanisettes" are completely sanitized after each use. Try one. Even my dignified British no-longer-young sister-in-law used one and lived to tell the tale. Plus ..... it is free! A great anecdote for your friends at home.

You shouldn't have to dig. There are right on your phone. On my Android I have "Toilets in Paris" and "Toilets in Paris Offline".
If you happen to have an Iphone, then I see "Public Toilets in Paris" in the Appstore.

When I'm out and about and need "Les toilettes," I go to the nearest cafe, order "un cafe" and ask "Ou sont les toilettes, svp?" If it is urgent, I explain in French that I need would like a cup of coffee, but first I need the toilettes. I've never had a problem. But be prepared that in those old buildings the toilettes are often down a narrow steep staircase and might be "unisex," which usually means private cabinets, but a common wash basin area. I always buy a coffee, at least, if I use a cafe's facilities. A demitasse doesn't fill up ones bladder.

If you go to a cafe and order something, ask the server for "un jeton" - a dummy coin that works to open the door. You shouldn't have to spend your money.

Sanisettes are free, located on busy sidewalks and now most are open 24/7.

Printemps dept store charges customers to use the WC. Galleries Lafayette and BHV do not. Other dept stores don't have facilities for customers.

...you shouldn't have to spend your money to open the door if you are a customer.

A pissoir is a stand-up item for liquids only. Very early public toilets for men only (a very small number of which are still around), simply consisted of a communal near-circular trough for liquids with a steel wall around it, with no other facilities.

Another good reason for a museum pass. If you have one, you can drop into a lot of museums that are covered by the pass to use the toilet.

I find it personally awkward to use the toilet in a restaurant when I'm not a customer. I used to own a restaurant and didn't particularly appreciate people coming in just to use the toilet and leave. I didn't bar them but now I prefer to buy something when I'm using their facilities.

I'd never go into a restaurant to use the bathroom, only a café.

The cheapest option in a café is to walk up to the bar and order "un café". The price - at the bar only - is normally 1€ or 1.20€.
Then ask for the jeton.