What to put in the field box?????
 

Can't believe this hasn't been covered yet...

A can of compressed air / dust remover: For blowing dirt out from the carb and needle valve, or other nooks and crannies.

After planting my plane in a tree, aside from fixing up the plane, I removed the engine scrubbed it down and removed the carb and rinsed it in alcohol. Two weeks later when the plane was back in flying condition, every flight was a dead stick stortly after take off.
Back home I went after stopping off at Radio Shack to get a can of dust remover. I blew a large chunk of dirt from the remote needle which I had not checked previously.
I had to wait until the following week before I could plant the plane in another tree.

Speaking of which: a 50ft pole for retrieving planes from trees if your club doesn't have one... the chain saw method might not be popular with the locals...

What to put in the field box?????
 

Pen or pencil and something to write on.

I have some "social cards" I print out on my home printer on Avery buisness card stock with contact info for me, the AMA, my local club, etc. And the backs are blanks so they serve well as scrap paper to write notes to myself about things to check in the shop later.