Think outside the box. For example, if 'duck' is on your list, it could refer to a rubber ducky, a wooden carving of a duck, or a roast duck on someone's dinner plate.
Visit the salon at regular intervals during an investigation. Sometimes finding new clues will open up new questions for Poirot to ask.
All of the items to find are historically accurate to the early 20th-century time period in which the book took place, which requires a bit of an adjustment in thinking. For example, if 'car' is on your list of items to find, you should be looking for an old-fashioned Model T Ford type of vehicle; if you're looking for a music stand, expect to see an old ornate wooden one instead of the black metal variety; and a bathing suit will be a modest black affair, not a sexy bikini!