For my younger, I keep diapers, wipes, formula, bottle, snacks, toys, sometimes a change of clothes. For my older, I keep a pullup just in case, juice, snacks, small toys, and sometimes a change of clothes.
My diaper bag has like 2 diapers, half a pack of wipes, 1 shirt, a pack of yogurt melts, a bottle with water in it, a small container of formula. I'm pretty sure that's all that is in it. My son is 10 months. We usually just leave it in the car anyway unless we know he's going to need to eat or be changed.