My niece is three years old and is showing some real interest in music. What is a...

I was going to say a drum lol, but then i see you don't want to drive her parents nuts. What about enrolling her in a gymboree music class? A recorder is also an easy instrument that a kid can play.

Sorry, I know that this isn't what you've asked, but why don't you spend the money to enroll her in a term's music class instead of buying an actual toy? I'm not sure what your budget is though ... perhaps it's a bit expensive.

If you really want to get a toy, I'd suggest a small keyboard maybe? Or some music / dancing DVDs.

Well, you could try a child-sized ukulele. It's very simple to learn and it doesn't make the kind of sound, like, say, a trumpet, that will drive parents nuts, just a nice, quiet, plinky-plunk sound.

Good luck!... ?