Great idea! use two Lack wall shelves from ikea and stagger them for wall stairs at $49 each, this can be done for under $100.
DIY cat whack-a-mole game. Take a box, cut holes in it, and cover it with shelf/contact paper. Take a pole or stick and tape a mouse toy horizontally to the stick. All you have to do is move it from hole to hole and they will go crazy