Out of all the things in the universe you could possibly have around, the 1/8-inch to 1/4-inch stereo plug adapter probably gives you the best tradeoff of anything i can think of between possible usefulness and difficulty of carrying around. (Never mind all that babbling in the Hitchhiker's Guide to the Galaxy about towels). It's basically just the metal head to a 1/4-inch stereo plug with a hole in the back for you to plug a 1/8-inch stereo plug into. So just slip one of the adapters onto of the 1/8-inch plug for your headphones, and suddenly you can plug it into anything that expects a 1/4-inch plug. They're pretty easy to find if you spend any amount of time looking, and they tend to come free with most headphones. I keep one of these things in my backpack at all times, and you'd be surprised how often it comes in handy.

The Radio Shack part number for a 1/8-inch to 1/4-inch stereo plug adapter is 274-363.