Your Nintendo DS isn't just for playing games — it can make music too! The Korg DS-10 ($40) is a great game to unleash anyone's musicality. It's a fully functional synthesizer, sequencer, and drum machine that allows you to flesh out a new song and save it for later.
And while Electroplankton ($100) isn't exactly in the "music making" category, you can make some amazing sounds and beats by playing with the games' 10 marine creatures, each with its own theme. Since this game is originally a Japanese title, try checking out Gizmine to see if you can get it any cheaper!