Asklemmy

Your knees and elbows. Train some kickboxing on how to strike with them.

Most items carried can be taken and used against you.

A portable alarm with a high intensity strobe can give you time to run toward safety. Or a smart-watch with a duress/emergency feature, to sound alarm and call assistance. There may not always be the chance to pull out an item though.

Your body size matters, get some expert advice on what will actually work for you, at your current size and fitness.

Confidence, if you prepare and walk tall, your chances of being targeted diminish.

Sturdy flashlight.

Used to carry a tactical pen disguised as a ballpoint pen.

Like people said, the best answers are the boring ones.

Best defense is being deescalating and avoiding dangerous situations, second is fast legs, third best is going to the gym, fourth best is probably pepper spray, and I would say the fifth best is a knife if legal. But most fights are over in under a second and there are no winners but only survivors with less severe injuries, and then it's almost always weapon > muscles > luck > technique > stamina.

Oh yeah and don't forget fighting is never cool. Everyone looks dumb, everything hurts and you will almost always regret it if you are sane.

Steel water bottle with one end narrowed to be a convenient handle –

A fat water bottle can't be gripped. Bad handle = bad weapon –

https://canemasters.com/collections/the-every-day-walking-canes/products/every-day-cane

Roll of nickels.

Here even knives, sprays and batons are illegal, unless definable as tools

Best defence is to run fast with good stamina.

If that's not an option, dog poop bag and round stone size of a golf ball. You can claim it was inspired weapon.

...or a two sided monkey fist with ballbearings that you can claim was a stress toy.

shovel to dig a trench

A ring on your keys, big enough to use them as a type of brass knuckles/ flail. Slide your finger in, make a fist and swing away.

A walking stick, maybe with a steel spike on the bottom for grip. Could be weighted.

Flipper Zero?

Caribiner like brass knuckles, sharpened toothbrush won't show on metal detectors, keys on a ring I between your fingers, anything can be made a weapon in the right situation. Bear spray, taser, knife, anything literally. But assume anything you present in a fight can be took and used on you.

The best advice is always to get out of the situation first and drive or run away. Good cardio serves a purpose in both fight and flight.

No knifes - but something blunt and small. Like a flashlight or mace. Made of metal, wood or even hard plastic. Could be retractable.

I carry a pocket knife mostly for functional purposes, but it also can be used defensively if required. A locking blade mechanism is definitely important. I guess a box cutter could work too.

A good pepper spray. Anything from POM for example.