To help students understand how timpani can be tuned, demonstrate the effect of membrane tension on pitch. With a coffee can drum (see lessons on Untuned Percussion), 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.
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. Students may enjoy making a “paperphone.” Cut rectangular bars of decreasing size out of colored construction paper and glue them to a posterboard or blank sheet of paper. Label the bars with the appropriate note names.
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.
Check out this recording to hear some great tuned percussion: 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 concepts for creating music lesson plans about percussion instruments.