keys: very handy for when you need to unlock the door to your apartment. If you add car keys you can also drive somewhere which is very helpful. I use mine pretty much every day
cellphone: it's like a multitool with a camera, flashlight and a phone. If it's a smartphone you can all use it to access internet. Save my life many times when I was bored and had to look at some videos online or check the news
wallet: great for carrying cash, ID and credit cards. It's good to have your ID with you when people ask for your name but you live in a foreign country where no one speaks Polish. You can use cash or credit cards to buy other stuff you need like a bottle opener
A tiny keychain tape measure. It's one of those things you have no idea how much you'll use until you have one on you all the time.
Looking at something online? How big exactly is it? It says 16.5" but can you picture that in your head somewhat accurately? It's just easier to pull out your tape measure and see.
I've also started carrying around a small flashlight everywhere I go. I have an Imalent LD70 Mini. It's small enough to fit in my change pocket and it'll put out a blinding ~4000 lumens on turbo (for about a minute before it gets too hot). I do admit that it's not the best flashlight ever. It has a ridiculous magnetic proprietary charger that will short on whatever if you leave it plugged in but it does what I need it to do.
I was the guy with all my keys and fat wallet with all cards and phone + Multitool knife.
Then I got inspired by my sister that travels a lot and tries to optimize for what she REALLY needs.
First I realized that most of my wallet is useless. I started carrying a smaller flip style one with my most used cards and some cash.
Then I reduced my keys to a carabiner with sorted keysets on keyrings, so I carry only the needed ones. Home + bike + cellar for daily use. Home + family keys for vacations. Home for going out. Added a very small key knife and a bottle opener that is part of my daily driver but left when not needed/forbidden.
The current setup is a phonecaee with space for 4 cards and maybe 3-4 bills. And my key setup.
Yes you have to remember to take some Special thing. But usually I have my old wallet in my backpack when traveling or when needed.
Leatherman. I carry a Wave. Got a Gerber and a couple others, but the original (this replaced my 1st gen) is best.
Pen & notepad. Always having your own pen that is kept full, and some paper to write someone a physical note, is an amazing superpower.
Face masks. COVID is still a thing, don't be dumb. Also carry hand sanitizer or alcohol wipes.
Flashlight. I have a small tube 9-LED that's usually in there, but also a 1-LED watch battery thing for the keychain, and I've got a few more around here. Yes your phone has a flash that can be kept on, but it burns battery, and is often awkward. Having light whenever is magic.
Switchblade, survival knife, folding knife, etc. Defensive uses, but also heavier utility knife than the Leatherman.
Pepper spray, derringer, etc. if you live in a dangerous area.
Survival kit. This is more of a backpack item. Get a tiny survival, first aid, etc. kit, there's a million uses for having some cord, bandages, shiny foil blanket, etc.
Cigarette lighter or box of matches. Even if you don't smoke, being able to make fire is another magic skill. I keep this with the survival kit.
Headphones, I have slightly-better-than-earbuds tucked in my coat. Noisy annoying people just stop being a problem.
And then my phone, keys, wallet, shades. Yes, I buy clothes with a lot of pockets.
I don’t know about pockets, but my bag usually has(besides the usual stuff) a tiny sketchbook with a pencil. I use it as an alternative to browsing my phone whenever I have time to kill.
Mostly because I feel I spend way too much time on this thing.. like now..
I have foldable scissors that are always attached to my keys. They're useful for, well, cutting stuff.
Safety pins. Not only that you can use them for clothes (for example if your coat's button decided to detach), you can use them for self-defense if you can't have weapons or pepper spray.
A small notebook and a pen. You can write down important things or just doodle out of boredom.
I have a tiny led light that fits on my keychain that I find really useful. It's better than the light on my phone and is a good backup light when I go camping. I also really like the Victorinox Rambler knife. It has a Phillips screwdriver, bottle opener, and scissors.
Everything on that list was added because I needed it and didn't have it on multiple occasions. There have been plenty of other things that I would love to carry, but found too bulky to justify.
A note to everyone recommending condoms: they do expire, and can wear through if carried in an environment with friction (wallet, pocket, etc). I prefer to play it safe and make a stop at a drug store should the need arise.
Leatherman Skeletool and a bullet space pen pretty much cover all my day to day needs. I used to carry a bootable flash drive with all the software tools I used frequently but haven't needed that on the go since I changed jobs so it lives in my desk at home now.
I’ve found a Gerber Dime to be really handy when I can’t carry my Leatherman. I usually carry a lockblade knife, multitool, and Fischer space pen. Also have a USB drive on my keychain partitioned with NTFS (bulk storage) and Tails with persistence.
I've carried a Swiss Tech Utili-Key on my keychain for 20+ years now. It's got a knife, a flathead screwdriver, a Phillips screwdriver, an eyeglass screwdriver, and a bottle opener. It's the same size and shape a regular key and security never notices it.
They make belt buckles that double as bottle openers for roadside Topo Chico and tacos, or beer (not roadside, don't drink and drive).
I have a tough little metal USB drive with a size A jack on one side, whatever fits my phone on the other, and firmware to be able to plug into said phone.
Musicians ear plugs in a tough metal keyring pill case.
I was gifted a tiny refillable keychain lighter. I didn't think I needed it at first but it's turned out invaluable to always have some fire handy. I don't smoke so I don't normally have a lighter with me otherwise.
A Victorinox Manager. It contains a small blade, a bottle opener with a magnetic Phillips screwdriver, a pen, a nail file, scissors and tweezers, all in a very compact form factor.