If your old iPod is gathering dust, why not use pdPod to turn it into a drum machine or a synthesizer? pdPod is an iPod version of a free, open-source application called Pure Data, used by experimental musicians with a mind for math and too much time on their hands. Installing it on an iPod isn't easy, but it sounds like a worthwhile weekend project if you're a musician with an extra iPod laying around.
To get the pdPod software up and running, you first need to load the Linux-based Podzilla iPod operating system as a foundation. Once that's done, follow the instructions on the pdPod wiki and cross your fingers. A video of the results are shown below.