Inspired by @courtney__jill
  1. A receipt.
    This happens pretty often. Bonus points if it's the receipt for the book itself.
  2. Pen.
    All the time. Then it's right there for creating marginalia.
  3. The book jacket.
    Pretty much the only reason I put up with the book jacket.
  4. Bobby pin.
    Works well, actually.
  5. Gum wrapper.
    I can almost always find one.
  6. Boarding pass.
    Convenient when flying. That's basically the only time.
  7. An actual bookmark.
    I know.