Ride those little electronic cars at the front of the store. Set all the alarm clocks to go off at ten minute intervals throughout the day. Contaminate the entire auto department by sampling all the spray air fresheners. Challenge customers to duels with tubes of gift wrap. Leave cryptic messages on the typewriters.
Re-dress the mannequins as they see fit.
Amuse themselves with the automatic sliding doors by disabling the safety sensors and crushing small toys. Repeat Number 14 in the jewelry department. Follow people through the aisles, always staying about five feet away. Hockey games erupt from random isles, using the entire store as a source of pucks and as the playing field. When you ask a checker if their lane is open, they continue to stare at the laser beam from the scanner.