I did some playing around with the knock tutorial on the Arduino website and was inspired by the Zygote ball by Tangible Interaction to make a small balloon lamp that would change colour when you tapped it.
Parts used: 3 leds, piezo transducer (a small speaker), a 300 ohm resistor and a 1 Mohm resistor, some wires, and 1 white balloon.
I soldered the ground legs of the three leds onto one ground wire, and gave them each a positive wire. I soldered 2 wires onto the piezo speaker as well. This was all taped together and then squeezed into a white balloon.
At the other end, i soldered the 1Mohm resistor in parallel with the speaker, and the 300 ohm resistor onto the ground wire for the leds.
On the arduino board, the positive speaker wire goes into analogIn pin 0, and the other into gnd. The positive led wires go to digital pins 9, 10, and 11, and the other wire to gnd.
You can take a look at the code here. It worked pretty well, but the balloon died after about an hour :P.