Well, most Americans go through Frankfurt at some point and that's definitely "the place".

I don't know how much you have but if it is use a few bucks, you also might want to give them to the Lufthansa Charity (Help Alliance) They will gladly do it for you and bags to collect charity are on each LH flight.

Also, some stores once in a while run "pay with Deutsche Mark" promotions in Germany but for somebody not living in Germany, it's not really an easy catch.

Don't the "Landesbanken" take them as well?