In my old house, I'd just lure them into a big walk in marble shower with the premise of a shower together after they've had a couple of beers. The buzz and the urge to pee usually gives them the idea. Unless they are totally squeamish about it, which I hope you would have vetted out through earlier exchanges. To be honest it was a while ago that I've been so naughty, but I had quite a bit of fun in that shower!