A spare tire is definitely something you will want to keep in your car. You never know when you are going to have a flat tire. Having one in a bind will make your day a whole lot better.
Certain tools are helpful things to have in a car as well. A wrench, a flashlight, a shovel, tire gauge, tow straps, etc. I think a filled three gallon gas can is also appropriate in this category. Not only is running out of gas embarrassing, but it is also a time consumer as you are forced to walk sometimes miles to a local gas station. Running out of gas usually happens to everyone at least once in their lifetime. Keeping backup is a smart idea.
A first aid kit is another essential you will want to keep in the trunk of your car. The first aid kit should be filled with band-aids, first aid cream, cold compress, ace wrap, cotton balls, etc. It should also include a gallon of drinking water and a warm blanket in case of an emergency. Don’t forget about the jumper cables as well!
I can’t tell you how many friend’s cars I have been in that don’t have the owner’s manual in it. The friend may ask if I can help them change the time on their clock. I don’t know how and neither does she since the owners manual isn’t present and they always seem to make it very confusing. I’m not sure why people even take these out of cars. To me, it makes sense to keep them in the glovebox of a car.