I carry a powerbank and various USB C adapters so if someone’s device dies I can help them out. It is a lot cheaper and smaller than carrying around seven different cables.
On top of that I carry Tylenol and bandages.
I used to carry a full first aid bag but kept forgetting to restock on supplies.
Nail clippers. I’m prone to hangnails, so it saves me a lot of pain keeping them handy, but they’re also good for cutting small things in a pinch (tags, loose threads, etc). I keep lots of pretty standard stuff in my bag/car, but the nail clippers get used the most for sure.