This one is from my hero, Bill Nye The Science Guy! See Bill Nye's Web Site. You need a balloon and some vanilla extract. Put a little bit of extract in a balloon and then blow it up as you normally would. Take a whiff of the outside of the balloon. This is best done BEFORE kids come in because the odor left behind from just opening the bottle can be enough to prejudice some noses. Just like the cell wall, the balloon will allow certain things thru - the vanilla molecules - and something not - the air in the balloon.
You can follow this up with a description or demo of why He balloons are shiny Mylar and not natural rubber or latex. The He atoms are so small, they seep thru the latex balloon and it will deflate in a matter of hours. The Mylar is almost a solid to the He atoms and thus will keep them in.