I took one of the cardboard cup holders from Starbucks (the ones designed to hold four cups of coffee) and turned it upside down in Inky's playpen, before hiding a treat under one of the four cup holes. She had lots of fun pushing, lifting and trying to chew the cup holder in order to get to her treat.
I think hard mode would be putting it on carpet to make it harder to push around and get the treat that way!
Sorry for the quality of the images, but hopefully you get the idea!