I'd put everything but the kitchen sink in your field box. Take whatever you are comfortable lugging around and have room for. Here is some of the stuff I carry;

Spare props for each of my planes that I fly that day. Rationale: Nothing worse than breaking a prop on a bumpy landing and not being able to fly the rest of the day

Spare Glow plugs: Plugs only blow when you don't have a spare in your flight box.

Spare tire: Can't tell you how many times I or someone else at field has lost a wheel. I usually carry 2-3 tires, don't laugh, this has saved my %$^&* several times since we all know those cheap wheel collars are worthless.

Spare Wheel collars, servo horns, servo horn screws, CA glue, CA accelerator for field repairs, clear plastic tape for monocote covering repairs, spare clevises, assorted screws/nuts/bolts, 6-12 inches of fuel tubing, SPARE WING HOLDDOWN BOLTS, various washers

Couple of carb parts. I lost a needle valve and needle valve assembly off of my carb two weekends. I am glad I had spares from crashed engines in my flight box.

Manual and electric fuel pump. The manual pump is a backup if my electric fails or battery runs low. Electric fuel pumps (other than Sonictronic pump) are not known for longevity.

Spare glow driver. I recenctly had a glow driver fail on me and I was glad I had a spare. Glow driver soldered connection to center wire/plug came undone.

Field charger with charge leads

Tachometer

Spare fuel probe

Buddy cord

Spare antenna in radio case.

Important One: Spare Transmitter Fuse. For some reason, if I turn on my Transmitter right after a quick charge on a field charger, the fuse blows. If I wait a minute or so, it doesn't happend. Nothing worse than a blown fuse and no spare for transmitter.

#64 Rubber bands. Go to STAPLES and buy a pound of these for about 3 bucks. This is cheaper than a small box of bands from Hobby Store and you get about 4 times more.

Lead to fine tune the CG on a new airplane or replace lead that may have fallen off a fire wall.

Several Popsicle sticks: These are a lifesaver for field repairs.

Don't laugh but I carry a butane soldering iron and solder in my field box and some shrink tubing. I've helped not only myself but others at the field with electrical and mechanical soldering issues.

Handiwipes, great to clean fuel off of hands and/or wipe down airplanes after a days worth of flying when you realize you forget to bring cleaner and rags.

I carry a spare receiver crystal and transmitter module on another frequency. No need to worry about having waiting to fly because 2-3 other people are using the same channel.

Sunscreen--a must unless you don't mind sunspots and/or cancer when you age.

Foam padding/scraps. A million uses to place extra padding in fuel tank, prevent transmitter battery movement with stuffed padding, or replace padding in a fuse that may be soaked due to a fuel leak.

ZIPTIES- A million uses at field. Can act as a wheel collar, hold servo wires together, keep clevises from separating, secure stuff.

Spare 3 MM muffler bolts: TRUST ME, YOU WILL EVENTUALLY HAVE A MUFFLER BOLT VIBRATE OUT IN FLIGHT!

Tools: Hex wrenches (metric and standard), glow plug wrench, hobby knife or razor blades, diagonal cutters, mini screw driver set, adjustable wrench, etc...

Trash bag (aka, airplane body bag).

Optional Chain saw optional to cut down stuck airplanes.....

Optional 10 dollar, Walmart one-man inflatable raft to retrieve planes crashed into lakes/swamp/etc. :-)