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The short answer is: Anything you think you'll need!

The longer answer is: It depends in large part about the details of your trip. If you are planning to spend all of your time in cities, the things you'll need are significantly different than if you're planning a trip where you'll be spending your time hiking and camping.

Having said that, here's a good place to start for basic, all-purpose items that are good to always have along inside your car.