When stocking your trunk, jumper cables and a canned tire inflator are must-haves. Checking your car's spare tire is also a important; nothing causes a headache like bursting a tire only to find the spare to be equally deficient. For the mechanically inclined, having a small tool kit on board comprising of screwdrivers, pliers and a little WD-40 can be invaluable when dealing with a roadside issue.
It's also important to keep a good pair of work gloves and flashlight in the car at all times. Getting under the hood, changing a tire, or dealing with other mechanical fouls is much easier with protected hands and a well-lit area.
Fuel for the driver is almost as important as it is for the car. Keeping snacks onboard can curb road trip hunger, and save you time money by avoiding stops for costly and unhealthy fast food. Easily storable and nonperishable snacks such as granola bars, nuts, and dried fruit provide a pick-me-up and can help quell hunger driving between meetings or between states.
At the end of the day, there's no definitive 'must have' list for car essentials. Personal preference and daily activities will determine what goods can stay in the garage, and what are invaluable in the trunk. For some an umbrella is a must, while others always keep a spare stick of antiperspirant in the glove box for hot days. By remembering instances where you are caught thinking 'I wish I had brought that' you can outfit your and your car to be ready for whatever comes your way.