High energy snacks and/or MREs -high-energy foods are essential. Some nuts and some jerky are good ideas. For MRE’s you just pour some water in the pouch and they self-warm and are ready to eat.
Maps Keep a detailed map of the state you’re in in your car at all times – or even a current atlas if you have room. Don’t completely rely on a GPS navigation system.
Booster (jumper) cables These can enable you to get your car started with a dead battery (if a good Samaritan comes along) and also enables you to help someone out in a fix.
First aid kit (and manual) This can be vital if you’re in an accident and someone has injuries. Slowing down bleeding quickly can mean the difference between walking something off and going into shock.
Fix-a-flat If your tire has a pretty rapid leak, Fix-a-flat can often provide just enough to get you to a repair station. I recommend at least two cans.