I agree with Fi - it definitely depends on the age of your baby or toddler what you need to take with you.

When Jacob was a baby and vomiting like mad, we would normally have the following:

~ 8 nappies
~ 2 - 3 Cloth nappies (we called these 'spitters' LOL - spew rags!)
~ 2 changes of clothes
~ Half a dozen or so bibs (yes, we needed them, believe me!)
~ Change Mat
~ Nappy Sacks
~ Bepanthen or something like that
~ Wipes
~ Sufficient bottles/formula etc for whatever stage he was at plus a spare one
~ Blanket/Wrap
~ Dummy

I would always have a spare bag packed with these kinds of things in the car as well (we still do!).

Nowadays, Jacob still needs nappies, wipes, nappy sacks etc, but he also needs his drink bottle, snacks, change of clothes in case he gets himself filthy, suncream when necessary, dummy etc etc.