I don't know that I'd choose to do this on a wall, but cool idea just the same. DIY project using a variety of colorful magazine pages cut into strips. Pick a silhouette/trace an outline. Then cut up the magazine strips and tape/glue it all together.
They poked tiny holes in the pink/red balloons, had everyone choose the color balloon they thought baby was, and then everyone blew them up at the same time. Only the blue balloons blew up, showing that they were having a boy.