These two experiments help your students explore air pressure. The first is a teacher demonstration which needs practice.
Lay a wooden ruler on a table, so that about half of it is overhanging the edge. Take a piece of newspaper, fold it to A4 and place it on the end of the ruler that's on the table. Ask your students to predict what would happen if you hit the other end of the ruler. Once they have discussed it, quickly hit the end of the ruler with your fist. The end of the ruler will fly upwards, moving the newspaper. Repeat the experiment using an unfolded sheet of newspaper, making sure you smooth out any air pockets. This time the ruler breaks. (Do take care, as the ruler may splinter, and it is possible for bits to fly out.) When the surface area was small, there was less air on it, and the paper moved. When the surface area was large, there was more air on it, and this stopped the ruler moving upwards, causing it to break.
The second experiment could be used as a student activity. Give each group a metal bowl, a plastic bag that is slightly bigger than the bowl and an elastic band. Use the plastic bag to line the bowl, so the opening of the bag goes around the rim. Secure it using the elastic band. Ask your students what would happen if they tried to pull the bag out of the bowl.
Once they have made their predictions, ask them to try the experiment.
It is very difficult because when the bag is pulled away a region of low air pressure is created between the bag and the bowl. The air pressure outside the bag is much greater than the air pressure between the bowl and the bag, making it difficult to remove the bag.