It's kind of hard not to be flip with this -- my initial answer is "a big crazy mess"
1. A booster seat for El. She's 5 and small for her age. We'll keep her in a booster of some sort for a few more years.
2. my iPod. It's old skool. You know, for an iPod. It's like 3rd generation, but it does what I need it to do, which is play music and pod-casts I listen to a fair number of pod-casts on my commute and set them up as an "on the go" playlist. Then I can just hit play and listen until it runs out and not have to mess with it while I'm driving.
3. my GPS. I've lived in the Orlando area for 23 years and there are parts of town where I just get all mixed up. It took me more than an hour once to get to a birthday party that was about 20 minutes away. Most of it was driving back and forth through the same section of town.
4. Sporting Goods. There is a soccer ball rolling around on the floor of the passengers side front seat and there are Frisbees in the way-back.
5. An umbrella. This is because I almost never use it. Much to my children's chagrin.
6. Mittens/gloves. I usually have a pair in the glove box, just in case the steering wheel is cold, about 5 mornings a year.
7. A first aid kit. It has come in handy more than once, luckily it was just for small things like bandaids.
8. I used to always have dive rings/dive fish in the car. You never know when you're going to need them. Like when you find yourself at the pool with a person who is learning to swim.
9. In transitional seasons, I have a bag of sweaters in the back of the car. There was this time that we took an outdoor table at Outback and it was March and the sun went down and one of us had to make a run to Kmart in the same parking lot to grab sweaters for everyone. That was the start of the "Ready Bag"
10. "Entertainment" Not a snazzy DVD player. Baby dolls, pens/paper. Crayons melt, so we try to avoid those. The VReader. etc.