just a thought: a synthesizer to go. Battery-fed, size of a cigarette box, SMD ,integrated loudspeaker,..2-3 octaves, a modified cheap mp3-player with recording-function, record your musical ideas whenever you got them, wherever you have them,... nothing special, 1OSC, 1LFO, (1 ADSR), no filter,... this could be much better than my pot-theremin.. (one button for on/off, pot for frequency, NE555, 2 resistors, 1 capacitor) :)