Students will enjoy learning about how the timpani are tuned by exploring the effect of the skins tightness on its pitch. With a coffee can drum (see lessons on Untuned Percussion), pull on the edges of the drumhead to put varying degrees of tension on it, while a student plays a steady drumbeat on it. If the students have their own drums, pair them off and let them take turns repeating the experiment you just demonstrated.
Explain how the bars on the glockenspiel, xylophone, and marimba are laid out like the keys on a piano, with a row of “white” keys along the bottom and “black” keys (chromatic tones) along the top. Have students make 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.
Show the students how resonance amplifies the sound of a musical 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.
Here is a great recording to check out: Camille Saint-Saëns, Danse Macabre. The composer gives the xylophone a prominent part at 1:45 and 3:45 (may vary slightly depending on the tempo of your recording). It is meant to convey the rattling bones of a dancing skeleton.
These are just a few ideas for creating music lesson plans about percussion instruments.