I have always kept a Rubbermaid container of "necessities" in both cars:
- Package of wipes
- 5 Diapers
- Diaper garbage bags
- Pacifier
- Spare outfits (2 for baby, 1 for big kid)
- Tissues
- Individual sized servings of Goldfish
- Mini water bottles
- Powdered formula (the travel tubes)
- Crayons/Paper and mini magna doodle
I know it sounds "nerdy", but this way, I have been able to just grab what I need on the way into a store, restaurant, MIL's whatever without having to carry the world on my shoulders (literally). There was a time when we were finishing up potty training that my husband kept a mini potty in the truck too, just in case!
I use one medium sized messenger bag (Eddie Bauer) with a center pocket and a couple of pockets throughout. I have transitioned from my giant planner to a DROID (which I really didn't want but it is so much easier) and have streamlined my keys so that they are just what I actually need on a daily basis and can clip onto the key hook in the diaper bag.
Seriously. Don't carry all of that stuff with you all of the time. Have it with you in the car and just grab it as you need it! Don't forget to replenish though!