There is a new design store on La Brea in LA called Mix. I haven't even been in it yet, but I was totally struck by the use of positive/negative space in their parking lot mural. To me, this is something I would totally use in a mural for one of my kids rooms, were they not already so decorated as they are - maybe when the kids get older and tired of the current decor...
The outline of the tree would be so simple to make (a design novice could always borrow an overhead projector and project and trace the silhouette of a tree onto the wall, and paint it in later) and from there - any great object could be placed into the tree!