Qtips for getting dirt out of the carb on noseovers, extra rubber bands for the wing, plugs, 1/8" round file and a small flat one, extra props of different sizes for noseovers and maybe go faster or do better prophanging or?  Make sure the hole fits in the motor. Wheel collars with screws in instead of setscrews.  Make sure the box will not slide or tip over in the trunk and hit your models.  If you can't, make sure it fits in the front seat, or in a corner where it will do no damage to the plane when turning a corner.  I like the corded glow starter with an adjustable glow on the power panel so you have a glow that will actually start a motor, and the needle tells you when the plug is blown.  The little 1.2 v glowdriver toys are ok for a quick start on the field but not very dependable.  And like everyone else says, keep it light, hemostats, plugs, I have inch and metric foldup allen wrench sets, and a few screwdrivers for head bolts and servo screws, and a small adjustable.  Oh and a pen and paper to list things that you have tried and didn't work  or worked better, or for phone# etc.  Some guys have a two piec toolbox with a detachable starter and plug driver tray for running out to the field quick  or for pit stops.  Oh and a spare control line handle and clips.