What is a model you could use to demonstrate the process of diffusion?
Refer to the explanation.
You could fill a beaker with water and add a few drops of liquid food coloring. Let it sit (don't stir it) and after awhile the beaker will be a uniform color, due to the diffusion of the food coloring. You can speed up the process by using warm or hot water.
You could also spray some perfume or an air freshener and have students raise their hands as they detect the odor of the spray. Don't try to wave the spray or blow a fan. Let it just diffuse on its own.