Here are a few fun websites to try out:

Classics For Kids - Has some fun games including a music composer tool to play around with.

SFS Kids - This site also has a really great music composer as well as some fun practice on things such as tempo, rhythm, and pitch.

Music Theory - This website has lots of great resources in the form of lessons, customizable exercises, and some advanced tools.

Music Tech Teacher - Choose a  fun quiz to take on almost any music subject!