I got a silly, chunky, bright yellow Casio kiddie's keyboard a couple days ago for $2.99. It's SOOO easy to mess with! There's a couple of connectors on the main intergrated circuit--specifically the seventh and eight pin from the left on the main IC. When you connect a wire to them, and then tap it to virtually ANYTHING in the circuits, and you get weird, looping "music." Sometimes its one of the keyboard's built-in melodies with a couple of stray notes worked into it. Other times its some strange alien techno with lots of white noise. Other times, it's just a burst of a few random notes with a high-pitched whine.
Problem with it, it's fairly "dense", structure-wise. There's not a lot of free space to build on. I may have to build an extension box of some sort. Oi.
Anyway, tons of success with this one, and I have another two old Casios from the same batch! :)