Never know when you'll need a bag. Until I whip it out for use, it acts like a liner for everything else in the bag, and lets me divvy out supplies to other teammates.
Three lengths of parachute cord (3000 lb test - green, 1500 lb test - white) and some carabiners, just in case some climbing needs to be done, or something needs to be hauled or hung.
Monocular, tapemeasure, multitool, flashlight, earbuds, and handsfree earpeice. It's nice being able to measure/see/talk to things while out in the field.
Wallet (with ID, business cards, and a lawyer's phone #), pencil, double-sized permanent marker, mechanical pencil, lead, eraser, lighter and water bottle.
Walkie-talkie (the other is charging) so we can stay in contact while tasking about, my keys (with spare flashlight and flash drive) and lantern.
Poncho (for rain), Calculator, eBook (has various physics, engineering, and biology books, + maps and tables), and a bag with small metal objects and polymer clay for harmlessly sticking things to other things.
Waterproof, padded neoprene document case. Sometimes holds my laptop, but lately has taken to holding my notebook, pencils, etc.