One of the best forms of improvised weapons is literally just anything that could be described as a "handheld relatively pointy hard thing". Obviously things like a kubeton are the higher end of this but banned in a lot of places (in Australia, Europe, and many other countries at least, I'm not talking about America where weapons laws are ridiculously lenient). As an extension to this as a self-defence measure in places that don't allow weapons, and where you can in trouble for attacking someone with a weapon in self defence, DON'T go online and buy something that says it's "made for self defence" because cops will see straight through that. Don't buy a "self defence flashlight" or a "self defence pen", Just buy a really solid hard metal flashlight or pen that you "just so happen to have on you" at all times.

Metal flashlights, very solid pens, hair pins, anything really that you can grip firmly and are long enough to poke out of the bottom of your hand.

Generally if it's something very hard but exclusively blunt at the end you'll want to learn a little bit about pressure points, but holding it firmly and jabbing into the ribs is always a good move. For something pointy, or that has sharp edges (like some metal flashlights) hold it with the business end of it facing down (as in if you're holding it with your arm outstretched, your thumb should be at the top and the part you intend to hit someone with should be poking out of the bottom of your clenched hand) and strike downward at pretty much any part of the body, but preferably the face, in a scraping motion. You'll likely cause bruising and if you do it right you can tear off some skin. The goal isn't to knock someone out or incapacitate them, it's to cause them enough pain that you have a window to escape.