Banana Splits
I used to like to do this prank at a library, but you can do it practically anywhere. All the material that is required is a book, a dollar, or something else someone is likely to bend over to pick up and a piece of cloth that makes a loud ripping sound.
I would place the book or dollar out in the open, then wait for someone to come over and pick it up. When the person bent over, the moment they had stretched their pants to the fullest, I would rip a piece of cloth. It is amazing how many people would then reach back to check themselves out.