Many of our trips we do by car and we have came up a list of items we never go on the road without. Some you may find very useful, others may seem a little not in place, but trust us, they all come in handy once in a while.
Navigation system - I drive an old, beat up car that doesn't have its own navigation system. That's not a problem, nowadays everybody has smart phones. Either iPhone or Android, they have similar apps for navigating. I personally use an iPhone with Google maps. It's a good idea though to have more than one app for this purpose. I've had several use cases where Google maps wasn't handy and I needed to use another app like Sygic or Here to navigate myself through the city. Also have in mind that Internet may not always be available, download your maps before you go on a trip.
Charger - when you are on a long car trip having the navigation opened all the time will be pretty power consuming especially if you are using mobile data. Always have a charging cable with you so don't run out of juice and get stuck in the middle of nowhere without any phone to even call someone. If for some reason you can charger doesn't work just a bring a power bank.
Local currency - plan the route, know the currencies you'll possibly need to use in the countries you'll pass through. It's always a good idea to have some cash with you. You might not need it, you might not spend it all, but it's better to have a gun and not use one, than to need one and not have, as the old saying goes. I'm telling you this because in my region (Eastern Europe) there are many shops and gas station that have problems with their post terminals, ok gas station are usually ok, but shops and roadside restaurant are a different story. And if you don't want to be and awkward situation where you have a card, but the place doesn't have a post terminal to accept your money you need to be prepared. I also had issues with toll taxes in Greece, they don't accept cards, only cash.
Phone - in my case that goes with the navigation. Your case might be different. You should always have a phone with you. If you ran into an accident, it's better to call somebody than wait for someone to stop and help you or if they can't help you at least lend you a phone to call someone. And what happens if you haven't memorized any phone numbers? What then? This here applies for the charger as well guys. Be prepared if you need to charge your device.
Drinks and snacks - you're on a long trip, you have a lot to travel, you don't want to waste time looking for a gas station to stop, or a road restaurant or something like that to buy a bottle of water or something to eat. Buy some food and drinks before on go on the road, not a lot, just enough to make it through the trip.
Nevertheless, it's not a bad idea to stop every couple of hundred kilometers from time to time just to take a break and stand up from the car seat. I recently traveled from Blagoevgrad, Bulgaria to Vienna, Austria without any stops. It was about 1250 kilometers and I was beat up at the end of the trip. So have that in mind as well.