United Kingdom
Leave your main wallet at home. If you lose your wallet all you'll lose is the money you came with and the one card you brought for emergencies - much easier to deal with. In the meantime, all that other stuff (stamps/library cards/loyaltycards etc) is safe and sound at home.
We thought they'd be a great idea for keeping tabs on pre-teen kids - of course so did everyone else and all we could get was everyone else's conversations!