Ableton hosts an interactive music learning site. In these lessons, you'll learn the basics of music making. No prior experience or equipment is required; you'll do everything right here in your browser. 8yrs+ 

This is such a fun exploration tool for all ages! Running in-browser, this website can be interacted with clicking and even with a plug-and-play MIDI controller.  

This is also a great resource for relaxation and general creativity. 

Soundtrap is a free in-browser digital audio workstation.  You can use Soundtrap to record and edit  multiple live audio and computer tracks. Your songs will save to your account, so you can go back to them when you have new ideas.  Have fun!

An infinite digital modular rack. There is a free version that is open source and standalone.