Now that's a practical question that has a thousand answers, all depending on who you want to talk to! If you're talking to a 2-year-old, it's the potty. If it's a truck driver it's probably the john. If it's an 80-year-old lady who's very upscale and fancy, you'd better say restroom or she may faint. If you're in my house, it's just a plain old bathroom -- it does have a bathtub in there and I really don't care what body function you plan to exercise in there, so we need not get all spiffy and talk about details.
In school, we said lavatory because the teachers were allergic to function words, but that always sounded too much like a scientist's laboratory to me. My auntie tended to say "little girl's room" or "little boy's room" even when it was all adults, but that seems a little too cute for my taste. In England, they could all be depended on to understand "loo" if everything more polite failed. And here in the U.S. they'll understand "toilet" when all else fails, but that's pretty rude.
Only fancy ladies talk about a "powder room" or give the excuse that they want to "powder their nose" because we don't use powder makeup anymore. It's all creamy and liquidy. But I do hear that occasionally when old ladied like myself get together in large groups and they don't want to admit out loud that they need to go tinkle. I've also heard much vaguer terms like "the other room" and just "I need to go" with raised eyebrows or a glance in the appropriate direction, which embarrasses easily embarrassed people. Or I just say, "I'll be back" like Arnold Schwartzenegger in his famous movie "The Terminator" (and I mimic his Austrian accent and say it in as deep a voice as I can manage, but being a little old lady that's not very deep and being a Texan, my Austrian isn't very good). People usually miss the joke, unfortunately and they just think I'm weird. Sigh.
Then again you can do like the littler kids in school did and assign numbers to the functions (though why in heavens' name the teachers wanted to know THAT I can't imagine). I need to do number 1 meant pee and I need to do number 2 meant poo. And babies who can't count as high as 2 say wee-wee and ka-ka.
Oh, and one more. In the U.K. (England), in the airport, they labeled the bathrooms "W.C." which is short for "water closet" an expression which probably hasn't been used in its long form for at least a century! When I took French in high school, they told us that's what they use in France too, only they say "weh seh". And in Mexico, supposedly, they say "el sal de banyo" (I've spelled it more or less as they are supposed to say it) but nobody I've ever met from Mexico uses that expression! So much for what they teach in language class! When I taught English as a second language, I just led them all to the restrooms and pointed to make sure they found them!
I'm sure that's more than you ever wanted to know.
Teacher of English to speakers of other languages from Texas, where everybody has their pet name for that place in addition and teaches their 2-year-old a weird name for their bottoms too, so their preschool teacher will be sure not to be able to communicate that need until the child is soaking wet!