Create Music
These tutorials are listed in order of difficulty, more or less, but feel free to try anything that looks interesting!
With CodeHS's Hour of Code with Music activity, you can create beats and color displays to go with them!
Using Trinket, you can use letters to enter musical notation, and embed your interactive creation anywhere on the internet!
PencilCode will walk you through a series of tutorials to learn to jam with code!
With EarSketch, you can use Python or Javascript to create musical compositions out of short snippets and sound effects - like GarageBand but with coding!
Create a jam session with Scratch by choosing and coding instruments!
Remix a jukebox with Tynker - switch between blocks and Javascript to try new things!
Sonic Pi is a tool for live coding beats and tunes - composing as you play! You'll have to download it, though.