Pluck Strings is a fun math-based virtual music maker. You can use this free web version or download it as an iPad app (which is both fun and educational for kids!).
You can rearrange the strings, select more or less strings on the screen to choose from, and change instruments.
From the site:
"No tricks or audio files are used: everything is calculated from raw sine waves, with a theory of how energy is released in a pluck.
- The vibrations are modeled based on how far down the string you pluck from.
- The velocity of midi notes are mapped to both pluck position, as well as pluck strength."
It's difficult to play an unpleasant sound on this app, making it a fun and meditative way to expend some energy and play with sound digitally.
See also: Guitar made from an old shovel