First option always is to actually play your instrument. Explore your method book, any of the new music you may have received, or access any interesting tutorial videos on your instrument that can be found with a quick Youtube search. Do something that interests you, encourages you to learn something new, and keeps you playing!
Other worthwhile options:
musictheory.net : Thorough beginning to intermediate level music theory lessons. Web-based.
Winnipeg Symphony Orchestra digital learning hub- Get to know more about the world of orchestral players and our local musicians.
Rock and Roll Hall of Fame database- Learn about the history of pop and rock music via multi-media resources.
Acapella App- Create your own multi-frame music videos.
Better Ears Beginner- Train your ears to hear intervals, tuning, and different components of musical structure.
McCracken Band- Free MP3 recordings, conductor scores and individual instrument sheet music for a variety of music, easy and difficult. If you are looking for an easy start up, always look to Grade 2 or less Band music.
National Arts Centre Orchestra Masterclasses- Learn from the pros who play in one of the worlds top musical ensembles.
Chrome Music Lab- Compose songs or draw musical pictures with this awesome interactive app.
Mixxc DJ Your Way- Free downloadable DJ software with controllers and effects.
Making Music Fun- Free online lessons for piano, recorder, flute, clarinet, trumpet. guitar, percussion (and bagpipes).
Breathing Gym Series- Learn how to take in more air and breathe efficiently with the help of Sam and Patrick. Many Youtube tutorials from these famous dudes.
Additional resources can be found at the National Music Teachers Association (USA professional organization) "Websites for Kids" compilation.