Call of nature
We have a lot of ways of saying “urinate,” or “defecate”.
This can mean anything from “wash one’s face and hands,” to “fix one’s make-up,” to “straighten, retuck, refasten, or smooth one’s clothing.” It covers all the things one is likely to do in a bathroom or WC.
Visit the facilities.
Same story: vague enough to cover anything that happens in a room where there is a toilet and a sink. You might say, “I need to visit the facilities. Where would I find them?”
Powder one’s nose.
Strictly for women, this one explicitly refers to make-up, yet it is widely used even by women who don’t use makeup. It’s a cover-up! Can be used anywhere to cover any purpose in the little room with the porcelain fixtures.
Go to the bathroom covers both visiting the little room as well as the acts of urination and defecation themselves. Though it is less polite and less vague than the expressions above, it is safe to use in front of almost anyone, though it’s far more likely to be heard in North America. In the UK, go to the WC is similarly used. (Americans understand “WC” but they don’t use it much.)
Use the toilet has pretty much the same usage, though for many Americans, toilet refers specifically to the white, water-filled porcelain seat that you sit upon in a bathroom and not to the room itself.
Sufficiently euphemistic that it can be used in a non-giggly way by patients and doctors when discussing the body. No one will be embarrassed by too much detail, and yet it’s less formal and less clinical than a word like “urinate”
Go to the little girls’ or little boys’ room.
Used by adults talking to children but also used by adults talking among themselves and not often in a joking way. Adults probably remember it being used by their elementary school teachers, who are masters at finding ways to talk about the bathroom so that mobs of children don’t giggle.
Go number 1 or number 2, to urinate or defecate.
This is also part of the language of teachers, parents, and children. Children might say to a teacher, “I need to go number 2.” Then the teacher knows how long the child should be out of the classroom and whether or not to come along.
Use the potty or go potty means to visit the bathroom or to urinate or defecate. This is language used when talking to children or in the presence of children. Similar terms are take a pee-pee or poo-poo, go pee-pee or poo-poo, or, for urination only, go wee-wee (British and American), to tinkle (British and American), to widdle (British), and to piddle (British).
Plain old go pee and go poop (without the second syllable repeated) are fine to use around children and are used among family members or close friends of any age.
See a man about a horse is an adult way of saying, “go to the bathroom,” as in, “I need to see a man about a horse and then we can hit the road” (I need to urinate and then we can leave).
Evacuate one’s bowels.
Used by medical professionals and police when making formal descriptions. There’s usually a notion that the bowel evacuation was not intentional, as might happen during a car accident or in the case of diarrhoea.
Take a piss. To urinate.
This is crude and mainly used by and among men. Not a good one for polite company, nor is take a dump, which means to defecate. The British take or take a slash (urinate) has the same kind of usage: it’s used mainly among men and boys.
Similarly, take a leak and drain the lizard aren’t really all that polite and not likely to be used by or among women except in a joking way.