I'd like to make a midi instrument sometime, using solenoids to strike a xylophone or even a full piano.
I'm in the process of making a full MIDI piano right now. It's an old pianocorder player piano which I've tapped into. The microcontroller solenoid driver works, and I just finished wiring up the MIDI input. The only thing left is a little glue code, and in a few days I'll be able to drive it from the computer.
I'm very interested in a MIDI mallet instrument, as I built my own marimba a few years back. The hardest part for me is sourcing a few dozen cheap solenoids.