....a 'convenience' bottle! (Especially when going cross-country in a glider).
Also usually take my passport and flying licences; only because a mate from uni once got given a really hard time by the RAF MP after landing out at RAF Cosford....he had no ID on him at the time.
Usual others; dunny roll, cash, cheque book, camera, aeronautical chart, pens, sunglasses plus spares, drink, nibbles, 'phone, coat (it can be a long wait in a field).
Barney