I usually just carry my headset around since all charts, flashlights, fuel strainers, et cetera typically stay in the King Air.
However when my flight bag was in full effect it housed to following:
1. Headset
2. Approach plates
3. Sectional and Lo Enroute Charts
4. AFD
5. Flashlight
6. Pliers (to lossen pesky overtighened oil dip sticks in trainers)
7. Dry erase markers
8. Pens / Pencils
9. E6B
10. Calculator
ummm.. i think that's about it