An excellent question. Beside the obvious of way too many boxes of flies:

Polarized sunglasses
Flashlight
Nippers (on a zinger which also functions as a tape measure)
Spools of tippet material 3x - 7x
Paste floatant
Powder floatant/drier
Shot
Hemostat or fine tipped needle nose pliers
Ketchum Release tool
Fishing License
Thermometer