A battery-operated mobile phone charger (for when it goes flat just as you are trying to fix PPR or get a METAR).
ID (for when Security jobsworths shout "Oi, you")
A tabard (to reduce the chances of Security jobsworths shouting "Oi, you").
A Mars bar (nourishment whilst detained by Security jobsworths whilst they flex their muscles making telephone enquiries about you).