At one time Thomas Cook (in the UK, anyway) were usually happy to exchange small amounts of money, including in some cases coins. I don't know if they still do this.

Of course, it was hardly worth it if the amount was too small, and sometimes you would have made nothing if you were to do it. Selling or passing on to friends or acquaintances is probably the best way then. For notes, if it's a readily convertible currency, you could put it in (for example) the donations box in a big museum.