A euphemism which I don't hear much nowadays is "the facilities." Another way of asking for the restroom is to mention that you'd like to "wash your hands." The tacit understanding is that you wish to wash your hands after using the "Facilities." I have also seen references to "the necessary room," although I believe that is an archaic 19th century usage sometimes put on the wall of restaurants trying to justify their expensive food and drink with ambiance.
In the book "Cheaper By the Dozen," the parents of the twelve Gilbreth children invented two euphemisms for the "necessary break" when taking automobile trips. They were "Visiting Mrs. Murphy" and "Examining the rear tire." I have no idea where they originated, but both Mr. and Mrs. Gilbreth refused to allow their children to enter the restrooms at the gas stations they encountered (this was in pre-Depression America, not long after the Great War). At that time, most gas station toilets were poorly maintained and filthy outhouses, assuming they didn't just point to the nearby woods and warn you to look out for the poison oak.
There is a toilet cleaner commercial now showing on TV which lists close to two dozen different names for that marvelous invention, which I occasionally refer to as "indoor plumbing." I know, that also includes all the water-using appliances in the house along with the attendant pipes, valves, and manifolds, but when someone mentions the potty, that's the first thing which pops into my mind. It's one of the reasons I never liked hunting.
