Maxwell's wheel is the classic way of showing the interplay between potential energy and kinetic energy, along with the effects of nearly-elastic collisions.
Maxwell's wheel consists of an axle-mounted flywheel suspended by nylon cords at both ends. Wind it up to the top of the frame and let it go! As it drops, gravitational potential energy changes to kinetic energy of the wheel's descent and rotation. When it has unwound to the bottom, the sharp jerk on reaching the end of the nylon cords reverses its linear momentum (while maintaining its angular momentum). This "collision" at the bottom is nearly elastic - some of the kinetic energy is lost. The ratio of energy kept to starting energy tells you how "elastic" the collision was through its coefficient of restitution.
Just be careful to keep the frame level and the cords even when you suspend the flywheel.
Dimensions: 17½" high, 11¾" wide, 4" deep.