Anything left over from 70-plus countries, even currencies no longer in use. Last year I eschewed my old bag-of-everything in favor of country-specific 6.5"-by-3.5" envelopes. Obsolete money is a nice souvenir. In-use money goes in its envelope with transit cards, so when I head to Hong Kong I just pull that sleeve and I've got cash and my Octopus card.