Foster's Home for Imaginary Friends: Destination Imagination (2008)

A toy box appears at the front door of Foster's and Frankie is ordered to take it to the attic. When she looks inside the box, she gets sent to a strange world which has an imaginary friend that does whatever she wants. Frankie decides to stay because she never gets any appreciation for all her hard work. Bloo, Mac, Wilt, Eduardo, and Coco set out to rescue her.