What to pack in a changing bag?

Nappies, wipes and sacks
Change of clothes
Muslin and bib
Changing mat
Breast pads if nursing, or formula, expressed or pre-mixed bottles
If you’re going to formula feed and mix formula on the go, get a thermos that you’ll use especially for formula water.
Calpol and sudocream
In a few months, I’d take a rattle/teether

I also have things like hand sanitiser, then depending on season sun cream/sun hats or wooly hat/extra blanket etc. Oh and emergency food/drink for you (particularly if you're bf), and a couple of reusable bags for the shops. And a small umbrella.