Binoculars. You will be traveling across lots of landscapes, stick a pair in the sidedoor of your car so if you stumble across a nice vista or see a cool animal you can get a better look!
Slippers so you don't have to wear shoes, make sure you have 'radio.garden' app on your phone so you can Bluetooth to your car. Sandwiches, simple home-made that needs no refrigeration for days, and a water bottle. Trash bags, small ones, and your chargers.
Don't rely on being able to stream content. Huge parts of the US don't have cell coverage. Have a library of downloaded music, movies, TV shows, games, books, etc. prepared
Snacks - stock up at the grocery store, you'll spend way more across the states in gas stations
Advil/pain reliever - you'll get headaches staring at the road for that long, and muscles will cramp up
Water. So so so much water. Soda is fine but bring a few gallons of water
Get a good charger system for your phone, and make sure you can hook it up to your car reliably
Oh and most important, bring peace of mind by getting your car checked before going. Go in and ask for a full inspection, tires, brakes, fluids, belts. I had a belt burst 8 hours from anywhere and had to sleep in my car, just get that stuff checked now
If someone wants a fantastic fantasy audiobook, the His Dark Materials series is voiced by a full cast and narrated by the author. It’s a really awesome series!
The advice here is good, but it may be a little too safe.
If you want an adventure, use an atlas and truck stops like Loves or Pilot for snacks, a nap or shower. These places are rad, super safe and clean (the stigma is bullshit). It can be a little more expensive and take a little longer, but there are a LOT of great and interesting people in the US and it's pretty much the safest it's been since the sixties.
Also, I recommend favoring AM or FM radio (in that order) to hear some wild stuff out in the boonies.
Edit: Make sure your car's lights are all working so the cops don't waste your time. The US may be safer than it has been, but the cops sure ain't.
I used to do a lot of cross country driving. Here are my recommendations:
AAA membership if you don’t feel comfortable changing your own tires, a good lug nut wrench and bottle jack if you can do it. There’s a lot of areas where you’re “in the middle of nowhere” and it can take some time for any support to get to you.
Make sure your spare has air.
Audiobooks, podcasts, and playlists downloaded. There may be long stretches of no service. If you can download the Google Map or Apple Map for your trip, it’s not a bad idea.
Non-messy snacks, gum, and a small cooler of drinks if you want to save money. Gas station food/ drinks add up.
There are a few apps to check rest areas/ bathroom recommendations. I liked Trucker’s Path but it’s been a while since I’ve travelled like that so there may be others/ better ones.
Get a dash cam before you go. There’s a lot of idiots out there. And the more you’re on the road, the more likely something could happen.
Plan a little time to make some extra stops if you can. There’s a lot of neat stuff you can do on your way that you won’t know about until you see signs for them. Or check Google Maps for places to stop on your route. Get out and stretch your legs somewhere fun.
And when you start to get hungry, look for good local spots to stop and eat. There are TONS of options that are so much better than the chain restaurants. Look for 4-5 star places in the area on Google Maps.
So I agree with most of the comments before. When I was young my family used to pile in a car and drove for a couple of days to visit family. Man, if you’ve got kids in the car you want to be sure you’ve got things for them to do. Unoccupied kids (speaking from experience) are distracting. We used to play the “license plate game”. We’d look for novelty plates and try to decipher what they mean, and look for each state plate too. Not sure if your driving alone or with someone.
I’d also recommend a couple blankets, towels (paper and fabric), a flashlight, and a first aid kit, and a roadside emergency kit too. Bring some cash, and have a backup payment method. Also if alone, make a plan to checkin with family at least daily. Let em know where your at, and where your headed.
Have fun and stop at cool sights and little touristy places too! Take lots of pictures. Wish you a safe and fun trip!