Buy loads of catfood and watch her face as you push it through the checkout. She will undoubtedly say something along the lines of "Got a few cats then?" to which you can reply, "no, I have a restaurant and use the catfood as gourmet pate." Watch their face fall.
Things to upset your local checkout girl, no. 24
When she asks you if you want cashback, ask for an obscure amount like £28.63. The blonder girls will look puzzled.
Things to do out on the street to wind up the public, no. 254
Look up at the sky in a busy town centre. Get someone else to join in if need be. Count how many people join in. Point at nothing. Count how many more join in. Walk away, with a straight face (very difficult).