Piano: Hoffman Academy online lessons are free but if you want sheet music and other materials you need a premium membership. Way cheaper and more convenient than actual lessons, though! (And a very good program. I feel like my kids are going to have decent musical literacy - not just able to read music but have some idea what it's doing)
Guitar: we introduced with
http://www.kidsguitarzone.com then moved on to Justin Guitar (this is a huge site, takes a bit of learning your way around. At the moment DS is working slowly through the beginner course)
Solfa (sight singing): Children of the Open Air on YouTube, and sightsingingschool.com
Jam.com Secrets of Drawing course. When we first signed up, we decided we couldn't afford it and cancelled at the last step. It asked why and made us an offer we couldn't refuse
- just a tip
When our subscription runs out, I'll be looking at the YouTube sketching course recommended above.
CM picture study materials from ahumbleplace.com
https://ahumbleplace.com/cmpsa-all/ (free)
Circe institute have a number of podcasts, including
The Play's the Thing, going through all of Shakespeare's plays one act at a time (this is intended as a teacher support, or you could listen with an older teen. Lots of fun.)
A Poem A Day (suitable for everyone)