music-box piano and many helium balloons (2007)
Attach a long ribbon to the wind-up ring beneath the piano (so piano can be retrieved from the audience afterwards). Attach balloons to the three little knobs on the piano lid. Finally, tether the little piano to the inside of whatever grand piano you're playing, a strut for example. If possible, this should be done in advance, earlier in the concert, and the music box should not be visible to the audience until it is released.
Before you cut the tether and free the piano to float off into the hall, wind it up and start it playing, but I suggest you conceal its sound with some other sound as you release it, a glissando across the higher strings, for example, might be just right. With luck the audience will quiet down till they can hear it.
Download a .pdf score.
Article in the online magazine Drunken Boat that includes the score and additional photos!