Making instruments with the human voice
The human voice is a pretty versatile instrument, and many skilled vocalists have been able to do convincing imitations of other sounds, and we've all heard "human beat box" artists work with a microphone to do great sounds.
That got me thinking, could we train a choir to work together to sound like anything, starting with violins, and perhaps even a piano or more?
The idea would be to get some vocalists to make lots of sounds, both pure tones and more complex ones, and break them apart with spectrum analysis. Do the same for the target sound -- try to break it up into components that might be made by human vocal cords with appropriate spectrum analysis.
Then find a way to easily add the human sounds together to sound like the instrument. Each singer might focus on one of the harmonics or other tonal qualities of the instrument. Do it first in the computer, and then see if the people can do it together, without being distracted. Then work on doing the attack and decay and other artifacts of the start and end of notes.
If it all worked, it would be a fun gag for a choir to suddenly sound like a piano or violin playing a popular piece. Purer tones like a flute might be harder than complex tones. Percussion is obviously possible though it might need some amplification. Indeed, amplification to adjust the levels properly might help a lot but would be slightly more artificial than hearing this without any electronics. Who knows, perhaps a choir could even sound like an orchestra playing the opening to Beethoven's 5th, something everybody knows well.