I know most cars nowadays have wireless charging, but trust me you never know. I learned this the hard way. Always carry a phone charger and laptop charger (USB/USB-C) if you will bring one.
waterbottle
First of all, it's sustainable since you will keep refilling it, and second, your car will be less cluttered. Let's be real first thing people start pilling up on are plastic water bottles. People even do this on a regular day in their car.
sunscreen
You should always have sunscreen with you, not just on roadtrips. You will be making a couple of stops so at one point you will be outside during the day, it's good to have sun protection so you don't get exposed to UV lights.
sunglasses
I know this might seem like a super common item, but trust me people forget. So make sure to carry a good one that lets enough light but also protects your eyes from UVA/UVB lights.
snacks
I know what you're going to say: "That's what the rest stops are for!" Well, there are going to be times that you're like really hungry, and rest stops with Starbuck, McDonald's, or Dunkin is not for another 10-20 miles.
Sheet masks
They are so fun to put on even on a regular day so for your road trip, this would be so beneficial for your skin. You will feel refreshed and hydrated and ready to take on the road.
totebag
They don't take much space and can be very useful when you need to use them for getting more snacks or when you spontaneously need to stop for a picnic or beach.