Pennies or M&M's™ can be used to simulate decaying atoms.

For pennies, students start with one roll of pennies in a box, all heads up. Shake the box and remove the pennies that have 'decayed' (flipped to tails).

For M&M's™ [local], students start with one bag of M&M's™ that have 'decayed' (flipped to label down).

Both activities involve graph construction and interpretation.