I've got a Kapoochi Milan satchel and I like the little compartments inside, makes it easier to find things. In my bag I have:

A traditional square cloth nappy for spews/spills, etc
2 sets of spare clothes
3 nappies
Nappysacks
2 Disposable change mats plus the change mat that comes with bag (I put the disposable change mat on the change tables in parenting rooms and then put down the bags change mat)
Antibacterial waterless hand soap
Sample size sudocream
Wipes
Spare dummy
And before I go out I will pack my bottle of water and a snack like piece of fruit etc