As the Nugget's grown, I find that I'm able to leave more "stuff" in the car and lug less of it in the diaper bag. Here's what I keep in the car:
Box of cereal bars or granola bars
Bottled water in case of emergency (we got a flat tire one August, and I was very happy that I had clean fresh albeit warm water for the Nugget to drink while we waited for AAA).
2 disposable diapers or pull-ups
Excedrin migraine (for me)
Hoodie for the Nugget - You never know when it's going to get chilly
Direction Notebook - instead of printing directions to our playdates each morning, I write down directions to each of our friends' houses and local parks/libraries and keep them in the driver side pocket. It saves paper and I can also jot down adjustments when Google directions don't cut it.
Bag of toys - right next to the Nugget's car seat. He's finally big enough to help himself to his own distractions - whoopeee!
Reusable grocery bags - I use my biggest one to hold all of them.
Cheap small playground ball
Towel - in case Nugget takes an unexpected dip or run in a sprinkler, also use it to wipe down slides at the playground after the rain.
Fold up picnic blanket - I got mine on special at Costco, but here's another version that zips up, is machine washable, is water-resistant side for damp grass, and has handles like ours.
Warm blankets - one crib size, one adult size, for emergencies
What's in your car?