If you have odd bits of money that are worth the bother, most money changers such as American Express will exchange paper for you (for a fee). They also collect coins for charities. UN Childrens' Fund perhaps.

I collect coins and have a interesting collection from my travels going back more than 40 years. I keep one each of a coin from a visited country. The others I try to spend at the time or put in the envelope provided on my meal tray on the home bound flight.

I am assuming that some countries will have persons who will stay be interested in US dollar bills.