Just obtain a tuning fork from a music shop or scientific catalog. Strike the tuning fork and immerse it a couple of inches into a glass full of water.
Sound is nothing more than vibrations moving through a medium.
When the tuning fork is struck, it vibrates rapidly. We cannot see the air molecules bouncing off of the fork because they are invisible to our eyes but we can see sound in something thicker like water. The splash pattern in water is similar to what happens in the air with sound bouncing off in all directions. If you have several tuning forks, have the students hypothesize how the pitch changes as the mass or size of the forks increases. I suggest you do this with the extension for the straw oboe where you look at the length of the vibrating column of air and pitch as they both relate to each other.