Its been a while, but when I actually had a nav bag....
1. A few instrument dial covers made out of Milo tin lids and fridge door suction caps, to cover up dead AH etc. (Cheaper than the $20 ones from Pilot Ripoff Shop.)
2. Tube of Coffee Milk, to stay awake with when you're getting a head start on tomorrow's F & D sheet; just don't suck all of it in one night or you'll be set for a 48 hour rave party.
3. ****loads of water
4. Couple of cans of John West fish, and tinned fruit; just keep the stuff away from the dogs. Use your ASIC card for a spoon.
5. Fencing pliers; the airport fence wire is your Aeromil parts counter on a stick....
6. Plug socket / 5/8" 7/8" spanners and plug cleaning gear; yeah and don't lend it out to someone....
7. DIY survival kit, with ELT, pen flares, mirror (and a lot more).
8. DIY med kit including a bottle of Betadine and some squeeze bottles of Chlorhexadine (kills everything so don't get it in your eyes).
9. Double looped 1/4" cable ties; one day you will have to tie someone up.
10. Camera, otherwise they'll never believe you.
11. Spare sunnies.
12. Empty condom wrapper to carry your paycheck.