The best thing to do with is to save it for a future trip. You'd be taking somewhat of a risk that the Euro and pound will have weakened against the dollar by then, but you also have the chance of avoiding changing a weaker dollar.

If you really want to change them, I recommend going to a currency converter like oanda.com, and seeing how many dollars you'd get for that bundle of currency at the bank rate. Then see what that bundle will bring you if you go to a bank, AMEX, or another forex service in the US, "what it will bring you" meaning how many dollars you will have left after the exchange at that institution's rate, deducting fees and, if you do it by distance, deducting also any shipping or mailing charges (which are just as much a transaction cost as a fee or commission, so it makes sense to deduct them.) My experience suggests that you'll end up with at least 15% less dollars than the bank rate.

Of course, you won't get the bank rate anywhere. But my experience is that at bank foreign exchange windows in Europe and Asia as an individual exchanging small amounts you end up with 3% - 6% less currency after the exchange than the bank rate. It seems only in America that banks and forex services look on changing small amounts for individuals as an opportunity for legal pickpocketing.