Something that works incredibly well at our house is the Car Toddler Kit. The kit is just a freezer size ziploc bag with several useful toddler items in it. I keep it ALWAYS stocked and in the trunk that way if I have to run a quick errand I don't need a diaper bag and if I go someplace with a diaper bag I have extras in the car in case I run out of something.
The toddler kit includes:
a change of baby clothing including matching socks
a burp cloth (even though we don't normally need them)
diaper rash cream & sunscreen
4 diapers & a travel pack of wipes
1 toddler warm up meal (you know like a gerber graduate) & a couple packs of snacks
an empty sippy cup and an extra pacifier
1 bottle of water
We keep a blanket in the trunk too because you never know when you'll have to change a diaper in a parking lot or want to sit at a park or when someone will be cold in the back seat and want a snuggle.