I remember Slasher replying to this sort of question a few years ago. Eye opening it was.
In mine.
Headset.
Pens.
Highligher
Folder with PLOGS
Folder with memo's/FCN's
Folder with handy hints/Allowance claim forms
Spare Weight and Balance graphs.
Some company stuff.
Wallet with Euros in it.
Thermos
Bottle of water
Little book with aerodrome phone numbers.
Stopwatch
Pilot's Atlas - Europe.
Rubber gloves (checking oils etc)
Cloth
Leatherman
Bottle opener
Licences
Logbooks
Sewing kit
Comb
Packet soup
Tampons. Absolutely essential!!!
Oh forgot, a minimag torch.
Memory like a sieve!! Must be craft.
Nav ruler/protractor/CRP 1 whizz wheel + little natty Jep style whizz wheel.
Just back from a trip and found even more stuff in there.
german phrase book
french phrase book
shoe shine thingo pilfered from hotel.
KFC wet wipe things
lip balm
calculator
clipboard (used as nav board)
I also carry a day pack for other stuff that are essential for late roster changes/overnights.