Ah, my old Fluxus group would be proud.
We used to do a thing we called "Dada Terrorisim" which wouldn't involve smuggling items into a store (although some people did some creative re-arranging of can labels - like replacing Hormel's Chili and Alpo Dog Food labels, etc.) but we would re-arrange merchandise in ways that were more useful to the average consumer.
For instance, we'd place Immodium AD next to the pre-packaged burritos, put No-Doz next to the sleeping pills (in case anyone wanted neutral alertness), desegregate the six packs of soda so that they weren't all of the same kind, and then of course just go for the absurd - heads of lettuce among the automotive items, Bottled Water in with the iced cream (which would be Bottled Ice after awhile), loaves of bread just about anywhere.
It's amazing how much you can get away with during very busy business hours. Most amateurs make the mistake of going very late at night, but that's when the store is empty and the people stocking shelves have nothing better to do than to hassle you. Better to do it during very busy hours.
My friend Kenny's favourite thing was cleaning and straightening a store. He'd go in and just go up and down every aisle during the middle of the afternoon facing the product, making sure everything was all nice and neat, chastising the customers if they messed up something he'd just neatened. He wore a blue apron just like the store clerks did (although his didn't have the name of the store on it) and everyone just assumed he worked there, including some of the people who worked there.
He'd use the break room, clock in and out (I don't know whose punch card he used to clock in and out, but I'd imagine that person got some extra money in their paycheque) and go home. He did this just about every week day for an entire summer once. He found it more satisfying than a real job.