I'm currently building a laser projector out of two hard drive VCMs.
As you are a beginner to arduino I would recommend something simple but interactive.
The simplest idea that engages people will go down a treat.
Perhaps look into motion sensors or distance sensors and then do sonething with LEDs or maybe a display or two. As it is for in a library, I would stay away from things that make sound. Perhaps a display which says things to people depending on how many people it detects/how close they are.
I look forward to hearing about what you do.
Mowcius