This is not really something you know nothing about, but something you probably forget about. You have your iPod or mp3 player with you but have nothing to listen to the music or books with. The simple answer? Ear buds.
Ear buds are generally not the most comfortable and you have to deal with wires (I cut a pair in two while doing yard), but they are relatively inexpensive and require no batteries. I keep multiple ear buds with me – my purse, briefcase and suitcase. What I like about the inexpensive ones is that it is easy to have multiple pairs with you.
Now you can go broke with high-end ear buds and blue tooth ear buds, and so it is a little more difficult to have multiple pairs of these types, but you can supplement your extra pairs with cheaper ones.
Again, there is nothing new about ear buds, but having extra ones can never hurt.