Making Music Fun is a great resource for free sheet music that also has wonderful music games and quizzes.
Dolmetsch offers an online musical dictionary:
Note-worthy's Printable Practice Tips
"This is Your Brain On Piano" courtesy of Vivian Handis at M. Steinert & Sons.
"Tracing Links Between Musical Training and Executive Function - and Bolstering the Case for Music in Schools" by Leah Shafer
TED-Ed video "How Playing an Instrument Benefits your Brain" by Anita Collins.