Just chanced upon this thread and thought I'd mention a couple more expressions familiar to me, but which by the looks of things, may not be all that widely used - perhaps they're just a regional thing.
As a euphemism for going to the toilet: πάω στο ιδιαίτερο
As a response to someone asking where you're going when you don't want to divulge that information: (πάω) στο γάμο του δεσπότη (which is of course a humorous play on the fact that Greek Orthodox bishops are [supposedly] celibate and don't marry).