Your car comes with a spare tire and a jack in the trunk, but you need to add a number of items that you just put in and forget about until the day you need them. Some things don't belong in the trunk, but up front, such as one of those battery chargers that plugs into the cigarette lighter (they are now small enough to fit into the glove compartment) and one of those pointed hammers with which to break the window and cut your seatbelt if necessary, should your car go under water. That belongs where either the driver or the front-seat passenger can reach it, in case one of them is not conscious.
So--what goes in the trunk? Most of these items are small--they're not going to take up all your luggage or grocery space. These are essentials, each one definitely worth the space it takes up on the day that you need it.
1. A blanket
Just put an old blanket flat in the back of your trunk and leave it there. If you get trapped somewhere in cold weather, you'll have it. If you decide on a spontaneous summer picnic, you'll have it. It can even be used for traction in snow or sand.
2. A roll of paper towels
Everything from a wipe to clean off your oil dipstick to napkins at that spontaneous picnic--and a thousand other uses.
3. Jumper cables
They're not just for you--you may need to get someone else's car started. Especially if they have stalled right behind your car so you can't get out of your parking space!
4. A plastic rain poncho
For unexpected rain, for an extra layer of protection against the cold, or even for a protective covering if you have to change a tire when you're all dressed up.
5. Emergency triangles
These lie flat on or under the blanket until you need them. They are bright-colored with reflective surfaces, good for day or night.
6. A container for small items
Vinyl or cloth if appearance matters, a cardboard box if it doesn't. You can put the jumper cables in it, but it's primarily to keep smaller items together so you can find them.
7. A can of motor oil
Consult your car's instruction manual for this and #8, buy the ones recommended, put them in the trunk, and they are there if you need them.
8. A container of anti-freeze
Probably too big for your container, so just tuck it into one of those awkward corners too small for luggage or groceries.
9. A gallon of distilled water.
In case you get stranded someplace where you are not sure if the water is safe to drink. Put it in the other awkward corner.
10. A crank flashlight
This small item definitely goes in your container. If you can find one with a radio that actually works for more than ten seconds on three or four minutes of cranking, get it, but my experience is that crank flashlights work, crank radios don't.
11. A first aid kit
Buy one ready-made at the drugstore, and put it in your container.
12. A knife
You know that big knife in a scabbard that's sharp on one side and serrated on the other? The one that looks like it belongs strapped to a Hell's Angel's boot? The one you've always secretly wanted but felt was "wrong" for your personality? Buy it! It can slice a watermelon purchased at a roadside stand, it can clean fish, and if you get a really good one it can even saw through thin branches, or through a rope that is too knotted to untie. It's even okay to get the one with a compass in the handle--indulge your inner boy scout, and put it in your container.
13. A can of WD-40
Get the new version with the straw that doesn't come off the sprayer! Put it in your container.
14. A roll of Duck Tape
With this and WD-40, you now have in your container the two items that keep the entire world working.
15. A can candle and matches
If you are stranded somewhere, you don't want to crank that flashlight for hours on end. Put the matches inside two plastic bags to protect from moisture, and put them in your container.