In addition to Minnreefers list, Icarry:

Popsicle sticks-for minor repairs, plus it's fun to get the part. The little wooden ice cream spoons are useful also.
extra wheel collars-to keep your wheels on, wheel collars have a way of disappearing
small tin of hardware (like an altoids tin) that matches the hardware for your plane
allen wrenches/ball drivers (standard and metric) that you need for your plane.

And here's my field box: