I have
2 nappies
packet of wipes
packet of nappy sacks (I use cloth nappies but just find these are easier than a wetbag)
change of clothes and extra socks
couple of bibs
teether
toy
couple of dummies
hand sanitizer
hand cream
change mat
bottle insulator (I use this for Darcy's drink bottle rather than for Nate)
peanut sling
hat
sunscreen

that's just as standard. I rarely use mine as we only live 5 minutes from anywhere but on a longer trip/day I will add extra nappies/clothes/food.

I leave mine in the car on a permanent basis, but when Darcy was little I used the nappy bag more and also kept another in the car with extra nappies/clothes/spew rags etc so that the nappy bag wasn't too heavy.