A bunch of solenoids controlled by a basic micro controller that can be triggered on and off via MIDI/OSC/Serial. The idea being that you can stick the solenoid onto objects in your surroundings (chairs, tables, dustbins, flagstones, people, etc) and then sequence the hits like a drum machine.
The hardware shouldn't be too difficult to make and it would be pretty extendible. If there are people wanting to go and do field recordings then it may prove a useful tool, hook up an old metal structure with 5 or 6 solenoids and make some interesting noise.
There are already plenty of people who've done this or something similar, some of them on a pretty massive scale (whole buildings hooked up and controlled by a keyboard) I'll post some links here when I get the chance.