I define an "improvised weapon" as anything you can get to within 3 seconds or less and employ effectively in a self-defense situation to augment striking power.
Let's break that down.
In many cases, when you need an improvised weapon, it's because you don't have another weapon, and you need something to help deal with an imminent threat, and therefore time is of the essence. That's where the "3 second" thing comes in; you need to be able to get to something you can deploy as an improvised weapon in 3 seconds or less.
Your location and environment have everything to do with the availability and types of improvised weapons. For example, in a restaurant, you're likely sitting at a table with salt and pepper shakers on it. Wrapping your hand around a salt or pepper shaker makes your fist much harder, and like the roll of quarters previous generations may have carried in their pockets, it makes it much easier to break someone's jaw. You can also empty the salt and pepper in your hand and throw it in the eyes of the threat as a distraction, and you may even momentarily incapacitate them. The knife and fork in front of you, a plate, a glass, a cup of hot coffee, even the chair you're sitting on can all be improvised weapons in a restaurant type setting.
In other environments like offices a simple pen is a great improvised weapon. Make sure it's a sturdily constructed pen, and hold it in your fist with the point sticking out of the bottom of your hand. Scissors and letter openers are also great choices, as is a fire extinguisher.
Pens are also great everyday improvised weapons that you can carry anywhere, even on planes while traveling. Flashlights fall into this category, too. A sturdy flashlight has been a favorite extra weapon of police for many years.
Another everyday item that is often carried on your person everywhere you go is a belt. A heavy enough belt can make a nice striking implement or be used as a garrote. And speaking of garrotes, your smart phone or tablet's charging cord makes a nice makeshift garrote, as does a power cord from a laptop.
There are too many different potential improvised weapons to list, but hopefully you got the idea from the examples given. The trick is to begin training yourself to incorporate improvised weapon identification into your everyday awareness and threat response rituals. Just like you would identify the exits and a couple likely escape routes whenever you enter a building, start taking a moment to identify likely improvised weapons near you. It can be learned and developed through training.