1. Hold on to it for the next trip.

2. If you know someone else going to the place that use the currency, sell it to him/her at the oanda.com rate.

3. Next time, when you are checking out of the hotel on your way out of a country (or currency zone) and you have lots of excess currency, use some of it to pay the hotel bill (keep enough for whatever needs you have on the way out, like bus/taxi/train fare to the airport).