Here are a couple of suggestions. Get a regular can of cheap shaving cream....like the Colgate stuff, for example. Put it in your freezer and the contents will expand and bust out of it's can. Peel off the metal and you will have a solid cylinder of shaving cream. Now, at quitting time, go hide it in one of the drawers in Linda's desk or filing cabinet. As it thaws it will begin expanding, and you would not believe how much this stuff expands. It will fill every drawer in the cabinet and spill out of every seam. Use a few if you really want an enormous pile of shaving cream everywhere.
Another thing you could do, depending on what type of desk she has is go in and take the drawers completely out of the desk. Now roll the desk completely upside down and put the drawers back in her desk. With the drawer's in the desk roll it back right side up, being careful not to let any of the drawers slide open at all in the process. When Linda pulls one of her drawers upen the entire contents will crash to the floor. Not only will it surprise the hell out of her -- she'll probably scream -- she simply will not even understand how what just happened could possibly have happened. She'll think, "How did someone turn my drawer upside down without everything falling out? How did they get it back in the desk?" And since it won't occur to her to have someone tip the desk upside down, she will proceed to just bite the bullet and pull out ever drawer and watch its contents spill all over her office floor.