I decided to make my nephew a mobile before he was born... and on his 11th birthday, I settled upon a 'theme' and materials (no, it was only about 18-20 months later). When it was finally finished, E. was at a great age for grabbing and pulling shiny things down upon his head. All in all, it was not my most successful gift effort.
I do still quite like it though, and the way the keys talked together was a beautiful surprise - not at all clunky or clanging but quite light and 'musical'.
Speaking of what constitutes music, there is an excellent sound-art exhibition on at Linden Gallery at the moment, called Cuttings. The installations are pretty interesting in themselves but I would very much recommend reserving a place for the performance aspect (3pm tomorrow! - Sat. 24), which involves two of the artists playing and interfering with the artwork, to quite magical effect.