In social situations: use the ladies' room (actually, I usually just excuse myself and go off to use it)
At work: use the ladies' room, use the restroom, take a bio break
With family: rest. This comes from a family in-joke - one day when I was small, my father pointed out that you dine in a dining room, live in a living room, bathe in a bathroom, go to bed in a bedroom... and I logically concluded that you rest in a restroom. So in my family, we say "Excuse me, I have to rest."
If I've been drinking and it's my first trip to the restroom since I started drinking: "I have to start down the dark path." My husband made this one up. You can usually put away several drinks before you have to use the restroom - but once you do, you're making frequent trips for the rest of the evening. So, "once you start down the dark path, forever will it haunt your destiny" (forgive me if I'm misquoting
)
OK, so I'm weird