Here is what I keep in my car emergency box:
Baby supplies (not pictured, extra outfit). Obviously, you won't need these if you don't have a baby. However, I would recommend having wet wipes and hand sanitizer no matter your current situation.
Tools: This is particularly useful for the times you want to move big things. Also, the jumper cables have come in handy! Fortunately, we've only had to use them once for ourselves, but we've also been able to help others quite a few times! We also have something similar to this air compressor/light that we've used SO many times.
Emergency Stuff: I'm not sure we need all the flares and glow sticks- they came with our car. I think it would be useful to have at least one of each, and especially make sure you have some source of light. The first aid kit is also very useful too. I would include a bottle of Tylenol or Advil. You never know when you will need it, and I've wished I would have had it in the car more often than not!
Misc: WATER is SO important to have. Napkins, plastic utensils, food, and a few extra garbage bags are also extremely useful. I want to get a few granola bars to add to this food. If I'm starving...I'm pretty sure Chef Boyardee is the LAST thing I want to eat. I had a bad experience during my last pregnancy. But, something is better than nothing! And lastly, that umbrella will ensure that you NEVER get caught in a rainstorm. I haven't used ours very often ever since it was left in the car. Murphy's law?
Not Pictured, but important seasonal items: Blanket, extra coat, gloves, boots. This may not apply if you live in a warm climate. Lucky.
Here is the completed box!
A few notes....like with the umbrella, it seems like you always have emergencies when you DON'T have things prepared. That is why I include the diapers. I can't tell you how many times I've needed one, and forgotten to put an extra in the car. Not fun. It's equally as bad when you forget the wipes! But, knowing you are prepared will help you feel ready for anything. Especially 'real' emergencies (diapers not included in this category): car accidents, or just car trouble! Also, being prepared also helps you help others, which brings its own sense of accomplishment, peace and satisfaction.
Obviously, you'll have to adapt this to your situation. But, my car emergency box contains things I already had in the house, it was just a matter of assembling them. If you have a chance to day, why not put together your emergency box, and then empty out your trunk and put it in? That way, you can have a clean car, and feel prepared.