1. Blow a balloon up to different sizes and hit it with the heel of your hand to make a sound. Blow the balloon up to different sizes to make different notes.
The notes change as the balloon skin is stretched to different extents, and so vibrates at different frequencies.
2. Blow a balloon up, then release the air from it while pinching the neck. Stretch and release the neck hole to make different sounds.
As the air passes through the neck of the balloon it makes it vibrate, which produces a sound. Different sounds are made as the neck is stretched to different extents.
3. Push a hex nut into a balloon, then inflate it. Hold the neck closed then move the balloon in fast circles to make the hex nut roll around the inside of the balloon. As it reaches a certain speed it resonates the balloon skin to make a humming noise. Try other objects in place of the hex nut e.g. marble.
4. Stretch a piece of balloon over a can, and hold it on (or fasten with a rubber band), to make a simple drum. Flick or pinch and drop the drum head to make a noise - the more it is stretched the higher the sound. The vibration of the drum head causes the sound. Vary how much the skin is stretched to change the note.
Blowing onto the drum skin also makes an interesting noise.
If the drum skin breaks leaving a strand of balloon across the top of the can, this can also be plucked to make a noise - vary the tension to change the note.