I personally like: Go to the restroom.

BUT

Here are a couple of others


bleed the liver; burn the grass; do number one; drain; drain the lizard; drain the radiator; drain the snake; drain the suds; kill a snake; kill a tree; lag; lant; leak; let fly; make; make water; nature stop; pass water; pee; pee-pee; pick a daisy; piddle; piss; point percy at the porcelain; pump ship; rack off; run off; shake a sock; shake hands with an old friend; shake the dew off the lily; slack; splash; spring a leak; squeeze the lemon; squirt; strain one's taters; syphon the python; take a leak; take a piss; take your snake for a gallop; tinkle; toy-toy; train terrence on the terra cotta; visit miss murphy; void; water the dragon; water the horses; wee; wee-wee; wet; whiz; wring out a kidney; wring the dew off the branch; wring your sock out
Not to mention
going to the necessary room.