Roadside basics
Reflective triangle or flares, a flashlight with fresh batteries, jumper cables, a tire pressure gauge, and a portable phone charger. None of these are expensive and all of them earn their place the first time you need them.
Weather kit
A wool blanket, a knit hat and gloves, and a pair of waterproof boots in winter. Bottled water in summer. Sunscreen and a hat are useful year-round if you break down on a sunny shoulder for an hour.
Documents
Keep current vehicle registration and insurance information in the glove compartment. Photocopies of your driver license and insurance card in a separate bag are useful if your wallet is misplaced. A pen and a small notepad help when exchanging information after a minor crash.
First aid
A basic first aid kit covers cuts, scrapes, and minor burns. Add any medication you take regularly. The kit lives in the trunk so it does not take up cabin space, but you should know exactly where it is.