A complete first-aid kit. Tuck in several doses of the family's must-have prescription medications, too. A cell phone. Police and auto associations now heartily endorse cell phones as your best safety tool. If you're in trouble, you just stay in the car and call for help.
Flares are smart, too.
A few tools. A wrench and a screwdriver are a good idea. In a pinch, duct tape can temporarily repair a blown hose. If you have an exotic foreign car and you're going on a long trip, you should consider carrying extra oil, fan belts and hoses. An auto association membership.