For the book cover I did some mock ups of how I would want it to fold and the two shown are the best and most likely the easiest to execute.
One page number one the unfolded poster would be this map. And on the map there are all these places and things the main charcters encounter in the book. When folded it you would see only portion of the map and unfolder you would have the whole map. This one is my favorite.
#2 I though about doing the character illustrations and on the front cover you would have the tw main characters and then you would unfold it and more would be revealed.
#3 my least developed. I had a hard time putting this one to paper. The Idea would be to have all of these important objects just scattered as the poster. and when you fold it up you only get glimpsed of them.