This helpful planning tool came to me after we drove to Florida last year (sniff, because we're not going this year). I made a car kit with handy things for the kids so that we could have quick access and not have to search through bags. Well, I realized that it would be a great thing to have all of the time, so the container is still there and I replace things as needed.
Here's what's inside:
A first-aid kit
This way if we forget the diaper bag, we're OK...if a child gets hurt on the playground, we're covered...and if the restaurant doesn't have coloring books (some don't believe it or not), then we have something to keep the kids occupied.
This has really worked for me! What's in your car kit? If you want to see more things that work go here.