Students will enjoy learning about how the timpani are tuned by exploring the effect of the skins tightness on its pitch. With a simple home made drum, you can often vary the tension of the skin by simply pressing or pulling at the edges of the skin, while another student hits the drum in steady rhythm. If the students have their own drums, pair them off and let them take turns repeating the experiment you just demonstrated.
Have a look at the keyboard percussion instruments, the xylophone, marimba and Glockenspiel, and show the students how they are laid out just like a piano. Students may enjoy making a “paperphone.” Students can cut out rectangular bars of continually smaller sizes out of paper and glue them to thick cardboard or a large sheet of paper. You can then put labels on the bars.
Demonstrate how resonance can amplify the sound of an instrument. With a xylophone or marimba you can remove one bar and try and play it with no resonator (pipe or box) underneath. Then compare the difference with placing it over the resonating chamber. You will likely need to experiment with bars and tube size before finding a combination that resonates well.
Recommended listening: Béla Bartók, Music for Strings, Percussion, and Celesta, III. Adagio. This movement has prominent parts for xylophone and timpani. The timpani performs glissandi throughout, an excellent demonstration of its pitch capabilities.
These are just a couple of ideas for creating music lesson plans about percussion instruments.