a number one: gotta (go) piss

a number two: gotta (go) take a dump
or if you think you're funny you can say "gotta go drop the kids off at the pool..."

EDIT: and if there's no bath in a bathroom, i would name this room a restroom or the (fe)male facilities

I have to get to the lil boys(girls if I'm trying to be funny) room

#1 time to hit the head/time to drain the lizard/time to break the seal(when drinking)

#2 i gotta drop the kids off at the pool

"I have to take a slash" is probably one of the most common terms used here in Ireland.

"I'm going to the jakes" and "I'm going for a hit and miss" are also quite popular.

My Kenyan friend used to say he was going to "spank the monkey" unaware of what the term meant. He knew what the monkey was but had no idea what spanking would do to it.

I have to go to the bathroom
excuse me
i have to drain the main vein
i have to download some data
gotta pee
gotta wiz
i have to drop off a few friends at the pool
i have to go potty (you can tell i have kids)
hit the head
gotta tinkle
oh, my bladder, be right back

im about to piss out a storm

gotta go feed the WC duck

the most common thing for me is "I need to leak" as opposed to "I need to take a leak"

I gotta go
gotta take a piss
going to the bathroom