Things that are more useful after an economic crash:
Just look at anywhere that actually has to deal with these problems. A shadow economy always rises whenever the cash money fails to meet people's needs. While there are some odd variations -- stamps are a popular prison currency right now -- certain things stand out as high-value for use and trade. When everything is suddenly in short supply, gold is valuable but you can't actually eat it and it won't keep you alive. Think about things with a productive use or a demand driven by craving rather than intellect.