Newton's cradle in the physics museum exemplifies a 1-d granular crystal composed of steel balls. The article http://scitation.aip.org/content/aip/magazine/physicstoday/article/68/11/10.1063/PT.3.2981 provides an overview of wave propagation and localization in mechanical crystals which might be readily constructed and observed to study bandgaps, solitons, create focusing elements, study breather modes. Hmm, try permanent neodynium magnet spheres or otherwise engineer new interactions? Add internal degrees of freedom? Going from cm to micron scale may support new practical applications.