I've got the BIG Sig field box (man, is that thing heavy!). Since I usually bring more than one plane, I have to carry some extra stuff...

Gal bottle of fuel for the .15 & larger engines

Qt bottle of 35% for the 1/2A's

Fuel bulb or fueler

Starter

Glow driver & charger (our field has electricity in the pavilion)

Tx & Rx charger

Spare TX battery

Allen wrenches

Couple of small screwdrivers

needlenose and regular pliers

Spare control horns, clevises, ball links, servo arms, wheel collars, etc. in small clear plastic boxes.

Single-edge razor blades

Medium CA & 5-minute epoxy, small mixing cups and craft sticks, and a couple of flux brushes.

Small zip ties

And of course, a few extra props...