A Blank Card with the size of one regular Bicycle Playing Card (88 mm x 63 mm).
You can use this card as a bookmark.
Use different colors, for differents cards.
If you are a magician like me, you can use this blank card for your magic effects (manipulation, prediction, …)
You can also do a remix of this card and add any value on the card, like “Two of hearts” or create your own gaff cards. I'm curious about your version :)