Notes

Wallet - Can be used to store the next three items on this list, though one may instead use a binder clip to save space.
Identification - Can be kept in one's wallet, it's often asked for when a person attempts to purchase alcoholic beverages. Law enforcement may ask for it if one happens upon a crime scene.
Bank cards - Another wallet item, for making purchases.
Cash - Sometimes, a place where one can make a purchase doesn't take bank cards.
Keys - To get in one's home, operate a motor vehicle, open one's safe, etc. Can be kept on a carabiner.
Multi-tool - Can come in handy in a variety of situations in which one may require a screwdriver, pliers, pick, knife, etc.
Pocket notebook - Sometimes, it comes in handy to have something to write on.
A pen - Sometimes, it comes in handy to have something that can write.
Flash drive - Can be used to upload a file to be read on your own computer, or store some files you may want to bring up on other computers.
Camera - Handy for memorable moments.
Watch - Having quick access to the time can help one cope with day-to-day challenges.
Cell phone - These have numerous uses, even if not connected to a network.
Handkerchief - There are many circumstances in which a small bit of cloth comes in handy. Without it, one would probably wipe their nose on their sleeve, or untuck their shirt to wipe their glasses.
Knife - Handy as a tool, and occasionally self-defense. A multi-tool may have this function.
Handgun - Perhaps a bit controversial, but it can help for self-defense.
Paracord - For instances in which strong cord may come in handy. It's easy to keep on one's person as either a bracelet or shoelaces.
Flashlight - A source of light can come in handy surprisingly often. Even a small LED with a coin battery can suffice.
Duct tape - Very handy item for repairs, a person can make a mini-roll to keep on themselves.