I leave the whatever it is in the book at the end. Not in library books, mind you. Don't like making more work for librarians. I started the habit when I found a baggage claim ticket in a book I bought at a used bookstore. I leave them in when I pass them on to the next owner.

I've found a few ersatz bookmarks over the years. The best so far was a 6 of clubs. The worst was a used q-tip.

I do this. I also have a kind of system to mark where on the page I'm at, depending on which way the receipt is facing / oriented.