I use a bookmark.

I work in a library, and holy smurf will people return books with any old thing as a bookmark! I have found birth certificates, legal documents, medical documents, credit cards, photos, money, a creepy business card that someone wrote 'I'm going to love you to death' on, CDs/DVDs, keys, photo ID, USB drives, and sanitary pads all used as bookmarks in library books...basically if it fits between pages, people will forget and return it that way. If it looks important, we will try and get the item back to the owner. I hate it when people write the due date on the bookplate, or markup the pages, that's just inconsiderate.

On that note, here are some items I've found returned in the bookdrop: Money, used syringes, a live mouse, a huge bag of weed, loads of books that belong to other libraries, clothing, etc. I wonder what will be waiting for me tomorrow X_x;;;