I think you kind of work out what suits your situation fairly quickly, but thinking back, we used to have a few spare nappies, a couple of plastic bags for soiled nappies, some wipes (when they're newborns I just used cotton wool, then progressed to the flannel wipes later when their skin wasn't so tender), a few changes of clothes and a little bottle of hand sanitiser. If you're bottle feeding you might want to consider how you'll sterilise that and what you'll need; if you're breastfeeding, throw in a couple of extra breast pads. Oh, and a spare square cloth nappy or two for unexpected vomit cleanups! You can use them to pop under bottoms for a quick change (our first nappy bag didn't have a change mat).