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), 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.
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.” Cut rectangular bars of decreasing size out of colored construction paper and glue them to a posterboard or blank sheet of paper. You can then put labels on the bars. This makes your music lesson plan much more fun!
Demonstrate how resonance can amplify the sound of an instrument. Take an individual bar from a xylophone or Orff instrument or something similar and strike it both by itself and over a a hollow tube (e.g., PVC pipe). It may necessary to have a go first with the instruments around the school first to make sure you can clearly demonstrate this using your instruments.
Check out this recording to hear some great tuned percussion: 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.