For this, I bring a tambourine, maraca, bones, tinwhistle, harmonica, jaw harp, conch horn, guitar, banjo and dobro. I also have a large rubber band.
Go over the 3 main families of instruments:
You can show vibration with a string (chimes aka harmonics are good here) or with a rubber band. (You can't hear it very well but it obviously vibrates).
I draw a sine wave on the board and say this is what a sound looks like.
Show amplification with a jaw harp (the mouth) or harmonica (the hands)
Then I draw a sideways "V" (or a large less than sign "<") and draw the sound wave bouncing inside of there getting bigger and bigger (which looks like it's getting louder and louder)
You need both vibration and amplification to have a good musical instrument.
Play If You're Happy and You Know It (clap your hands, pat your knees, shout hooray, do all three) and discuss how you've done all 3 main families: percussion (clap or pat), wind (singing), and string (if you're playing the banjo or guitar or something.
Explain the resonator on the banjo. Talk across the top of it to show amplification. Play Crawdad Song (they were studying spiders at the time and a this was about an arthropod).
Get out the dobro and show how the banjo resonator fits inside the guitar
body so it's louder than a guitar (play them both) but mellower than a
banjo then play a fast sing along (She'll
Be Coming Round the Mountain).