Paper towels, knapkins or possible a smallish roll of toilet paper. Not only to wipe up spills, some of the rest stops will have no toilet paper, towels or hand dryers.

Windex or other glass cleaner in a smallish spray bottle. Take some cotton rags with it. This is for the inside of the windshield. Even the normal thin layer of grime which is normal increases the glare especially at night. Clean glass makes driving easier and safer.

I usually throw a small bag of extra clean rags in the trunk with an extra jug of water. That is for face and hand washing. Sometimes it is nice to be able to freshen up a bit after a long drive.

A blanket and pillow. They do not need to be large, just enough to get a bit comfy if you want to nap while the wife drives, or vice versa. Sometimes it really helps on a long drive.

If you take a thermos or two you can have hot coffee or tea.

edit: IO almost forgot. If you are leaving at the end of September be sure to throw a couple of pairs of gloves in. You might appreciate them if you need to change a flat tire or something.