Medical passport with medication etc details,
keys,
public transport ticket/card,
tissues,
hand sanitiser,
Blackberry with ICE numbers,
driving licence,
tool card (handy credit card sized piece of kit with various tools on it)
pen
diary,
cash and credit cards.
We're a very crowded island. We could carry guns to deal with the tensions to which this gives rise, but we find pointedly ignoring each other a cheaper, less messy option.