Go with an equally mad friend and race each other around the shop on the trolleys.

Take all the cans out of the multi-packs (those cans that say "not to be sold separately") and move them to where all the individual cans are. Then complain loudly to other customers that the store is doing something illegal.

Take a jigsaw puzzle and, when you go to the till ask if you can have it reduced because it seems to be broken.