Gravol, in case a child becomes car sick (I was prone to this).
We would pack our suitcases for the entire trip but also have a
duffel/backpack with clothing, pyjamas, and toiletries that were being used for the road trip. This way we wouldn't need to open all the suitcases. It was especially useful when we went as a family, but my brother and I were adults. That's 4 suitcases of stuff. Only 2 fit in the trunk, so the other two were on the roof.
An odd one is a night light. Some hotels are VERY dark, and some kids might be scared, or if you're like me, you get up in the night to use the washroom. Having a night light to light the room a bit helps a TON, so that I don't stub my toe on something, ESPECIALLY if you're sleeping at hotels during your drive up.
Don't don't don't forget a child's favourite toy/blankie/etc! As a 9 year old, I forgot mine, and I was heartbroken for the majority of the trip during the night (and especially on the road), without Pongo to hold onto. v_v
Enough CDs/music to get you through the entire trip. I'm really picky about what I listen to, so I don't normally listen to radio stations. And this way you don't have to search for one if you need something to listen to.
A really good suggestion I received once when considering a trip I made with my BF and his son was to pack brand new toys/items for the kids that they only get during the trip. It especially works if they don't know they're going to get special items during the drive up. It'll keep them entertained.
Perhaps journals and pencils/crayons? When I went as a child, I enjoyed drawing what I saw out the window and writing something about it.
Our car always had snacks/treats - but not for the kids! My father was diabetic, and rather than having to stop to eat, he'd grab a granola. I'm sure it worked well for the kids, too, though. I myself certainly wasn't thinking about eating though. lol