A 30x30 cube of polystyrene.
Yes, I'm serious.
Need an assault boat? Need an insulated cold-weather shelter? Need to float the team Humvee across a deep river? Need to make a dummy vehicle to spook or decoy with? Warm up a bayonet over a fire and you've got it.
More practically, I've often thought of running a character named "D.J." - he's got a working (compact!) Panasonic boom box and a stash of lovingly cared-for cassette tapes of pop music from all pre-war eras (a crapload of "best ofs") that he keeps wrapped in a flak jacket and carefully stored in whatever ride they have.
He also manages to come up with working "D" batteries periodically when hooking in to the vehicle is out of the question.
He's a former AFRE disk-jockey pressed into service and he grabbed those items on his way to muster...