I grew up with a wiseacre who called it the defecatorium -- sure to raise eybrows.

My girlfriend in high school called it the, "tinkletower." First and last time I heard that one.

The best euphemism is 'euphemism' itself. As in this...

"Honey, the young woman guest at the nightmare dinner party in Who's Afraid of Virginia Woolf?, says she would like to powder her nose. George, the host, turns to his wife and says, "Martha, won't you show her where we keep the euphemism?"

http://www.robertfulford.com/Euphemisms.html