I have a couple of different diaper bags.
In the car, I keep a small "emergency" bag with a couple of prefolds, a snappi, a cover, some wipes, and a wetbag (actually a small camping "ditty bag"). This is the "oops I forgot the diaper bag" bag, or the "gone for longer than we planned" bag.
My diaper bag for longer outings usually has the following:
-3 or 4 diapers, pockets or AIOs
-bottle of water (good to drink or clean up big messes)