The notes at the bottom of the bag

Ranked by what you actually get back, which is often less than the effort.

Best to worst

1. Spend it before you leave. Full value, no spread, no fee. Fuel, a meal, gifts, topping up a transport card. This beats every other option and it is not close.
2. Keep it for next time, if you return to that country or region regularly. Notes do not expire, though very old series occasionally get withdrawn.
3. Change it back before departure, in town rather than at the airport. Bring the original exchange or ATM receipt — some countries need it.
4. Change it at home. Poor rates and many banks now refuse currencies they do not routinely handle. Worth it only for meaningful amounts.
5. Donate it. Airline charity envelopes and airport collection boxes accept coins and small notes, which is a better outcome than a drawer.
6. Sell it to another traveller. Works in hostels and traveller groups, at a rate you both agree on.

Coins are a separate problem

Almost nowhere buys back foreign coins. Bureaux deal in notes because coins are heavy, low-value and expensive to move. Assume coins are spend-or-donate the moment you receive them, and break notes deliberately near the end of a trip so you are not accumulating them.

Avoiding it next time

• Taper withdrawals as the trip winds down.
• Switch to card for the last day or two where cards are accepted.
• Use up cash on the predictable end-of-trip costs — the airport transfer, the final meal — rather than putting them on a card.
• Check whether the currency is restricted on export before your final withdrawal, not after.