A pocket knife and a small flashlight. You don’t realize how often you could use either until you have them.
My knife is an Opinel #8 ($20) and my light is an Aurora A33 ($20).
I carry a bag most of the time, so I’ve got a little extra room than most, but I’d probably still carry both if I didn’t. The pocket knife is the size of, well, a pocket knife and the flashlight is only the size of a sharpie.