Depends on what I intend to shoot, really, and what camera/lens(es) I plan to use for that. But usually besides the essentials of camera, lenses, extra batts, charger, triggers,
white balance target, microfiber cloths, etc., I keep these "oddball" items with me in my bags:
Folded up sheet of regular silver tin foil about 18" x 18"
Folded up sheet of black foil about 24" x 24"
a square yard of black velvet or black felt
a square yard of white rip-stop nylon
lengths of string and wire, several mini-bunjees
several feet of gaffers tape rolled onto a little wooden dowel
a roll of scotch tape
Half a dozen little (like 1" long) orange-handle spring clamps
UltraPod II (whether I have a regular tripod with me or not, this thing goes where I go most of the time)
bug repellent, band-aids
a couple
rainsleeves
Kimwipes are jammed into all kinds of nooks and crannies