Passphrases for credit card numbers, maybe? It'd be easier to remember "bobcat reissue onscreen crook" than a series of 15-16 numbers and you'd have essentially unlimited versions.
You'd have to limit yourself to a set of reasonably well-known words (and potentially weed out words that sound too alike). If we're very optimistic and assume the final set has 10,000 words, then one word has the same amount of entropy as 4 digits. Therefore four words and 16 digits are the same size of address space. You'd need at least five words. And you're quickly getting to the point where it's getting problematic to print these on the card in a font size that everyone can read (elderly citizens with bad eyesight included).
