Well there's also assets. The bank can hold the value of land deeds or house loans for example. The house and the land it's on are tangible things that can be evaluated. Though I suppose that's a degree removed from printed bills or minted coinage.
> I suppose that's a degree removed from printed bills or minted coinage
Yes, because the "value" of those assets is just a guess until you try to sell them. It might be a highly educated guess, but it's still a guess. And the time the bank would need to sell them--because it needs more cash due to an unusually high demand for cash withdrawals--is precisely the time that your educated guesses all go out the window: you're selling not because you're making a trade to improve your rate of return, but because you need cash, now, and buyers will know that and will gouge you on the price.
