At the hardware/plumbing supply store:
Walk up to a clerk in the plumbing section. Look nervously at the toilets and ask very insistently, "where do you keep the toilet paper?"
Point to a toilet and say, "Hey, I think that one's clogged. It won't flush." Then walk away and watch him approach the toilet.
Put on a shower cap, grab a scrub brush (toilet brush works too) to use as a microphone, and sing in the display shower. Bonus points if you start to collect an audience, even more if they applaud at the end.
Bring a basket of laundry and load a washing machine. Then, pull your clothes out, smell them, and complain to another shopper "This model's no good. The clothes are just as dirty as when I put them in."
Go to a mattress store in your pajamas. Ask a clerk to turn out the lights so you can test a mattress.
If you work in an electronics store, hook up the display TV's to a camera in the manager's office. Works especially well if you know he picks his nose or has other nasty habits.
