An observation: money
Going to England, I brought some British coins with me from the Strange Coins Drawer, leftovers from previous trips. It turns out that some of these are No Longer Money.
I'm not talking about shillings and sixpence from before decimalization, either.
I understand that Americans are strange about changing their money -- no obverse in common circulation has changed in since I was a very small boy -- but making money into something else seems dangerous, scary. You might undertake this in the service of some Big Idea. Europe is a big idea. But, do you want remind people that money can turn into metal disks?