I cloth diaper. Normally I have
1-2 cloth diapers
A few disposables "just in case" (I got locked out of my apartment one day -- so glad I had those!).
Wipes
Changing mat
Spare clothes
Nursing cover
1-2 wetbags (sometimes I keep 2 in there, clean stuff in one, and the other for dirty, helps me stay organized!)
A stick of CJ's BUTTer
Teething tablets
Bug spray (I could take that out at the moment, I'm sure, but when the weather was nicer and we were going to the park I wanted to have something safe for him!)
Sunscreen (same as bug spray).
Hand Sanitizer
Teething rings (those are his toys when we are out, lol).
My keys & wallet
Sometimes a water bottle for me
Granola bars for me
Random gift cards that we got at our shower, to places not nearby, in case I get a chance to stop in and get something for him I don't have to remember it specifically.
A pen
A small bottle of tylenol for me if I get the start of a headache/migraine to ward it off, otherwise I won't be able to function
A stun gun in an outside pocket so it's easily accessible (yes, I carry a stun gun! I've been approached by strange men before so DH insists I carry it)
Man, my list seems long! But it doesn't really seem like that much. Sometimes if I'm popping out to a nearby store I don't take it at all, besides the keys & wallet of course, and maybe a burp cloth. If I absolutely needed diapers, wipes or a change of clothes I could buy it. I don't always carry a burp cloth or receiving blanket but I used to keep them on hand more often, though they were usually with him in the car seat rather than in the bag itself.
The bag is usually stocked with everything on the list except the cloth diapers, as I don't have enough to keep a couple in there if I'm not going out. I need them at home! Then of course the wet bags, nursing cover, and any other things that need washed get removed and cleaned as needed.