Very cool! I went to China a few years ago but did not keep any currency from the trip. Not sure why since I usually do. I have Euros from a trip to Europe a decade ago (floating around somewhere), and every denomination of Guarani from living in Paraguay.
My family went to England for a trip when I was a small child and I'm sure I have some pounds floating around somewhere too. If my goal is to hold on to foreign currency for posterity I should probably be taking better care of it...