I drive a lot, I keep a few things in all of my vehicles. Some of the items stay in the trunk, others stay with me in my laptop bags.
- gallon of water, more on long trips. one behind the seat, the other in the trunk.
- food, like nabs and a few cans of something with an easy open top. If I'm traveling with the family or I'm driving through BFE I'll pack a bag full of stuff.
- blanket, cold weatherbag. I keep a blanket and a coat in my trunk "just in case". used the blanket several times to keep the child happy when traveling, the jack is an original patagonia synchilla that I don't want to toss.
- power inverter so I can use my laptop anywhere (very handy so the child can watch DVDs while we travel). I have this mounted under the seat with the outlet hanging out from under the middle of the seat.
- car chargers for my cell phone and iPod.
- laptop so I can reload my iPod no matter where I'm at.
- cash, I keep an assortment of bills hidden in my car, you never know when you might need it.
- jumper cables
- AAA card
- gov't ID, if I'm stranded I can be sure to get a sweet hotel rate.
- flashlight/candles
The jack/spare I'd never consider removing, coolant I kept when I was having radiator issues.
In winter I pack some warm clothing, extra food and a couple of Sterno. When I was working for Dominion Resources I used to travel from southern VB over to Yorktown. One day it snowed really bad and I got stuck on the side of the interstate. I normally don't wear a coat, just a shortsleeve shirt (I don't get cold). I was able to bundle up then cook some dinner on the sterno for me and another carload of people who were in front of me (they went off the road while trying to get around an ice covered tree that fell on the road). When I was driving my J20 to work and living way out in BFE I carried a chainsaw and some fuel along with a gallon of gas (not that it would help, the J20 was a damn pig).