I would suggest that you only use the type of ipod that uses a flash drive rather than one that uses a HDD (try a shuffle, nano or touch model) as the HDD drive has moving parts that could break and the flash drive models don't have moving parts. My second suggestion is you use one without a screen as it will be too hard to see while riding and again just one more thing to break in a fall.
I use a shuffle model with tiny speakers (slightly bigger than the AAA battery that powers them). The whole thing is small and light. I have several playlists set up in itunes to suit my differing moods, I just sync with the playlist I want for that ride.