A fun toy would be a musical instrument with a microphone that can detect and modify pitch in real time. So if you sing an A but play a B it will transpose your voice up a tone. I understand such things are common in modern recording studios but I'm not sure if they work in real time (besides which, I don't think it would be too difficult to create such a thing from scratch).
Better still would be if it was polyphonic, so you sing a note, play a chord and hear a trio.