...from the Washington Post Travel section this weekend:
What to do with foreign money after your vacation
Essentially, a list of reasonable options for spending the last bit of foreign currency in your wallet/purse.
In general, I never have that issue with Euros as they just get reused the next trip, but I have a lot of other paper currency (separate from coins) that goes into the same place we keep our passports, and brought out again if we revisit. Coins we generally toss in the "coin jar" and give to younger kids as "play" money.