If you are talking "real world"....I carry chapstick, a lighter, extra surefire, small bottle of breakfree, strobe light, at least 2 knives, signal kit(mini chem lights, small section of VS-17 panel), small LED light tied off to a whistle, extra IZZY bandage...that is pretty much it on first line. In the R.A.I.D. I carry extra water, energy bars, extra bulbs/batteries, large VS-17 panel, lighter, chem lights, 550 cord, extra compass, map pens, matches,.......oh wait....your talking airsoft....my bad.
I would bring some band-aids for minor cuts. Water. Snacks. Extra batteries/bulbs. LOTS of ammo. Maybe a camera for that "KODAK Moment". And gum. It keeps you from thinking of how hungry and thirsty you are.