I ended up keeping the few euros, pounds, and my oyster card I had left from my first trip to Europe as a souvenir. With a little luck I hope I'll be able to go back either next summer for the following and if I do I'll probably take it back with me and use it then.

It is. As we were starting to descend into Chicago they made announcements about envelopes being available for anyone wishing to donate any extra foreign currency they didn't need. Several people around where I was seated requested them which was nice to see.