Best Buy is the way to go. Up until the end of January, you can exchange any unopened game for full credit. Be sure to get the games stickered at the front and then just walk over to customer service.
Didn't try that, but you can just go to Best Buy to get store credit for it. That's what I did. Rode my bike from CC to BB and got $32 in store credit (they give you tax, too). I was hoping they'd have a copy of Mario Kart, but I guess I'll have to wait.