Being an adult is more than just reaching adulthood. There is quite a bit of responsibility that one must successfully handle to truly show they're fit for day-to-day life. Preparation is a key part of that. You can't do well on a university exam without studying, you can't get to work without fuel in your car, and you can't survive a freak storm without emergency supplies. All of these situations take proper preparation and execution. While I can't tell you what to have with you for every situation, I can give you some tips on what you should have on your person every day you leave your dwelling to make your life (and possibly others' lives) easier.
This is getting mentioned first as it is the most important. Many of us leave for work early and come home late, a flashlight helps us conquer those dark moments. You might think your phone is a good substitute, but phones typically have weak lights with short-range and an auto-turn off feature. In addition, your phone is fragile and its battery life is much better served for emergency phone calls than light. New LED flashlights from brands like SureFire are rechargeable, durable, bright, affordable, and have a decent range. I use them for walking down sidewalks, checking around my car when I walk up to it in dark parking lots, finding lost items on the floor, and there have even been a couple of times at my old job during power outages where I was the only one still able to work because I had a flashlight on hand. Flashlights also improve your personal defense, as few assailants want to attack someone who can see them coming and make them visible. A flashlight is a necessary item to have on you.
A pen and something to write with:
It is surprising the number of people who don't carry at least a pen or two on them. Even though many things are done electronically these days, a pen & paper are still important items to keep on you. While neither the pen nor paper have to be anything fancy, a nice space or waterproof ink pen and also a tearproof/waterproof notebook is helpful. These notes can be used to help you quickly jot something down before you forget, exchange phone numbers/business info, write information down with ease while talking with a phone up to your ear, etc. I've seen too many situations where difficulty was created because someone didn't have a pen on them, don't let yourself get sucked into that.
A watch, and no, not a smartwatch:
A watch is a great tool to add to your everyday life. Not only are they stylish and add to your professional appearance, but they are also convenient. Many believe watches are unnecessary now that smartphones tell time as standard. However, your phone has a limited battery life, and there are many situations you can't use a phone (university exams, meetings, etc.). It also is a stupid situation that you'd pull something out of your pocket to check time as if you were living in the 19th-century. I am counting against smartwatches as well since they also suffer from shorter battery life and again, those same situations you can't use a smartphone, a smartwatch typically isn't allowed either.
A good watch, such as any Casio G-Shock, is very durable, unlike the smart devices. Mine not only is waterproof, but it has also been bumped and dropped multiple times, including going through a dryer. Meanwhile, it solar charges, auto-updates time by NIST radio synchronization, and is 20-25% the price of a smartwatch. Get a good standard watch, and you will be able to wear it for many years to come.
A good multitool:
A multi-tool is by far one of the most useful things to have on you. While you likely can't fix your car on the side of the road with it, it's like carrying a simple toolbox in your pocket. I have saved a lot of time and come to the rescue by doing quick fixes with the screwdriver attachments. The pliers have helped torque some nuts down on loose bolts. The bottle opener is obvious in its usefulness and the filer has helped me clean up my nails. I don't use the wire cutters or knife very often (next section explains why), but I have used my multi-tool for over 2 years now on the job and in everyday life. A good multi-tool will save you from embarrassment and save time from having to get a toolbox for simple jobs.
A good pocket knife:
While most multi-tools have knife attachments, they are not meant for heavy-duty use. A quality pocket knife, however, can be used to cut many different things over and over before needing to be resharpened. Pocket knives are also one of those tools you likely will be finding yourself using quite often. Cutting through cardboard, thick plastic, and tape off packages is what I use them most for, but there are a variety of other uses such as opening letters, cutting cables, and even for emergency situations such as cutting the seatbelt off someone in a car accident.
When it comes to recommendations, my first is checking your local laws. In certain states or municipalities, knives a person can carry must sometimes be of a certain blade length or shorter, sometimes a certain overall length folded or under, sometimes must be foldable, and sometimes can only be manual opening, with no spring assist. Where I used to live, a pocket knife of only a 3" blade or under could be carried concealed, which is why I got the Para 3 above instead of the longer Paramilitary 2. Second, unless you work in a field such as Emergency response where you are likely to damage/lose your knife often, stay away from the cheaper 400-series blades in the $30-$50 range. Not that blades of this price from Buck or Kershaw are bad, but blades from brands such as Spyderco, CRKT, Benchmade, usually have a much more durable, strong, and corrosion-resistant steel for the blade, even if it's more expensive. CPM-S30V blade steel is by far my favorite grade of steel, and it is for many others (CPM-S35VN is also a great choice). Knives with this steel typically fall in the $80-$140 range but will last you for many years of use with occasional sharpening. It is an investment.
Even if you don't smoke, carrying a lighter on you can provide some benefits. First, you might save a friend or make one who does smoke by letting them get a light. I've ran into this situation twice already this past year. Second, they are good for emergency lighting. If the power goes out and some candles are around, you can easily provide some light to an otherwise dark space. I would put this on a lower priority than other items on this list, however.
A portable charger and very small charging cable:
In the world of today's smartphones, having charging cables and a portable charger is a good idea. You never know when you or a friend might need to get their phone some extra juice. This is helpful for marking emergency phone calls, keeping enough juice to catch that taxi or ridesharing service, capture the last photos of a party, etc. You don't necessarily need to carry these on your person, especially if you are smart about your battery use and charging habits, but have them at least in your car, bag, or workstation.
A small first aid kit:
Cuts, headaches, and stomach aches can happen at any time. You don't need to take a large kit around with you, but a small pocket-sized kit that contains things like bandaids, alcohol wipes, and common medicines for headaches, stomach upsets, and colds is a good idea. I have prevented the need for me to leave for home, stop by a store, or kept my coworkers bright-eyed and working strong from fixing cuts and getting some medicine to stop soreness or headaches right then and there.
Thank you for reading and I hope you learned something today, and consider adding some of these items to your everyday life. This list was not meant to exhaustive, and some things that people often carry on them every day such as mints/gum weren't mentioned. I did not mention firearms as that ability to carry varies greatly from person to location. Feel free to share your experiences and other items you'd recommend for carrying below!