A singers' chromatic pitch pipe. I sometimes learn songs off CDs in the car, then sing them to myself. The pipe means I can establish what my natural key would be for that song and I can start creating a guitar / 'tina accompaniment in that key (although that bit, I don't do in the car!).
Many years past, some scrote broke into my car and stole the stereo and all the tapes. I got the tapes back, though. I could track him from the car park - he had obviously been walking away, going "never heard of that, never heard of that, don't want that one...." I just followed his trail and picked them up again.
Lastly, my father bought a part-chavved Vauxhall Corsa when he needed transport, quickly. There's nothing funnier than listening to the Archers through a subwoofer system.