The Cayman Islands Dollar (KYD) has a good exchange rate relative to USD. If you can find a place which accepts foreign coins for foreign exchange, then you can buy KYD coins at a discount from a coin dealer, stack them, and exchange them for closer to the KYD-USD rate. It may take a while, but for KYD it's worth it.

For reference, KYD does better than GBP, and EUR does better than KYD.