Plan another trip to Europe. With some of the fares out there you'll do better taking more vacations and spending the Euros in Europe than you'll lose trying to exchange them back in the USA.

Big cities will have more options than smaller ones but otherwise you're pretty much at the mercy of the three options noted above (AmEx and your banks). Call and see what you can find.

Slightly OT, but why would one have so much cash in Euro left at the end of a trip? Seems somewhat strange to me.