Depending upon the take-home message you want your students to get, you might structure the activity in a few different ways, but the basics are the same.
You'll need some steel wool, vinegar, bottles or flasks and a balloon.
Pull apart some strands of steel wool and push some into each bottle. Pour some vinegar onto the steel wool. (Some instructions tell you to soak the steel wool in the vinegar for a few minutes and then remove the steel wool. I just left mine in it).
Stretch a balloon over the opening of one bottle, but leave the other as is.
You could find the mass of each system at this point, if you're interested in conservation of matter.
Allow the bottles to sit and the reaction to occur.
The vinegar removes the coating from the steel wool, and the steel will be begin to oxidize in the presence of oxygen.
As the reaction is occurring, the balloon will be pushed into the bottle. Why?
The oxidation reaction is using up the oxygen in the bottle, which will lesson the number of air molecules in the bottle, thus reducing the pressure in the bottle. Because the pressure outside the bottle is greater than the pressure inside the bottle, it will push the balloon in.
You can stop there if you're interested in simply looking for evidence of a chemical change, studying the chemical reaction or seeing the affects of air pressure.
If you're interested in conservation of matter, continue on.
Find the mass of each system once again.
The closed system (i.e. the one with the balloon covering the opening), should have the same mass it had in the beginning.
The open system's mass should have gained mass, as it continued to pull more oxygen into the system to carry out the reaction further.