I usually carry a few junk silver coins enough to pay for a cab to get me home should I find all my Credit and Debit cards stop working. That signals to me that something system wide is seriously wrong with the Payment networks. One may stop working but not all?
I find it not practical in carrying a big coin but it makes a great conversation starter.
I've found that already just carrying these ASEs in my pocket. When I accidently take one out and someone sees, it makes for some great conversation.