Donate coins you brought from foreign countries when you traveled there to UNICEF.
In the Narita airport, there is a donation box of UNICEF at a baggage claim area. I've donated a few times. Can you donate coins to UNICEF only at airport? No. You can do it, at an each branch of Mitsui Sumitomo Bank in Japan, and the Japan Committee for UNICEF accept donation by mail.http://www.UNICEF.or.jp/cooperate/coop_coin.html
I went to the nearest Mitsui Sumitomo Bank, and I found a donation box of UNICEF. I donate all of coins I have kept for next travel.