Music Express Magazine produced a series called The Music Show. It does a great job of explaining musical terms using characters and in a way that kids can understand. Here is the an episode explaining harmony.
Boomwhackers are plastic tubes of different sizes that people hit to play a variety of notes. In this video you will see the music group "Thud" from Harvard University playing the song "Crazy Frog". When they play notes together they are creating harmony.