Organize the students into groups of 2 or 3. Have the students fill the bottles with various levels of water. Then have them blow across the bottles and ask them what they notice about the sounds that the bottles make. They should notice that when there is more water in the bottle, the pitch of the sound is higher. If they have different size bottles, have them blow across the empty bottles and again ask them what they notice about the sounds.
Smaller bottles make higher pitched sounds.
Have the students experiment with making waves in Slinkies. Stretch a Slinky across a desk, and have one student hold down an end. The other student shakes the free end of the Slinky in various ways to create different wave patterns. Have them first try a single shake so a pulse bounces off the stationary end. Then they can vary the frequency of the shaking to create different harmonics.