If you have any foreign coins at the tail end of your trip, use them all first. Financial institutions will not accept foreign coins for exchange. The suggestion is to spend the coins at the shops at the airport from where you are scheduled to depart. Use them to buy snacks, postcards or a last minute souvenir or gift.
Some people keep one of each coin as a souvenir. But don't count on planning to use them on your next trip. Countries like the United Kingdom updates their foreign currency in interims.