I think it depends how old your baby is.
Newborns are more about change of clothes, lots of nappies, feeding stuff and spew rags. Toddlers not so much the nappies as their bowels are a little more consistent by then but lots of snacks and water to get them through a tired time when out and about!
Always a good idea to leave a few nappies and a change of clothes, plus a jumper in the car somewhere - just in case!