Coins are attractive options because they're common and their dimensions are relatively easy to obtain, but they are somewhat restricted to the country the photo is taken in. Rulers at the least are based on international standards of measurement.
Credit cards I'm not sure about, they all appear to be of regular size but I don't know if their dimensions are standardized.
CDs and floppy discs are also creative solutions, they're common enough objects to give a sense of scale, and they have to be of similar enough size to be accepted by the opening on a floppy drive.
Still for sheer precision and versatility, I have to choose the ruler.
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