The JelTone uses an Arduino to generate 12 musical notes simultaneously. The Arduino source code is attached (_12tone_synth.pde). You'll also need food-safe wire (such as silver), a small high-impedance amplifier - the little radioshack battery amplifiers work perfectly - and an audio jack or cable to fit your amplifier.
Cut the tray and the frame out of two 12x12 sheets of acrylic in bright contrasting colors, and glue the frame on top of the tray. Add some feet to lift it up off the table - we used 3 short lengths of 1" acrylic rod from the plastic shop's junk box.
Cut 15 pieces of silver wire, each an inch or so long, and stick them through the 15 holes in the tray (13 on the keyboard and 2 in the back of the piano), folding them over so they don't fall out.
Glue or otherwise attach the programmed arduino to the underside of the tray. Also glue on a 9V battery holder if you like.
Solder 13 jumper wires to the 13 silver keyboard wires on the underside, and run them to arduino outputs 0-12. Connect a wire from the ring or shield of your audio cable or audio jack to arduino ground, and connect the other audio wire to the two remaining silver wires. Also solder or clip two jumper wires with alligator clips here.
Now seal all the holes and fix all the silver wires in place with hot glue. I learned from experience: if you use hot glue first, it'll melt and burn when you try to solder!
Testing: turn on your amplifier and arduino. Touch one of the rear wires with one hand, and the keyboard wires with the other. The audio signal should be conducted through your skin from arduino to amplifier and you'll hear a tone!
For presentation: cut jello, fruit, or other conductive material into shapes and put them in the keyboard frame to form the black keys and white keys. Pile up your leftover jello and fruit in the back of the tray. Now you can play the piano by touching the salad pile with one hand, and the keyboard with the other. Or, clip the alligator clip jumpers onto a pair of forks or spoons and play the keyboard with those! You can see both techniques in the two videos.
Optional: cut out the retro-style logo from shiny shiny acrylic and put it on top of your fruit salad!