Due to a recent (few months

) lack of interest in being held over a toilet, I'm adding a folding potty reducer to our bag as soon as my order arrives.
We also bring:
water bottle
snack container
wipes (used pretty much never

but sooo important when they are needed)
couple changes of diaper (add in more for longer trips)
couple changes of pants
rags (mostly for my nose, but also great for small messes that aren't worth using the disposable wipes)
plastic bag or two for laundry, trash, as needed
and then pretty much anything we shed during the course of events ends up in there and gets cleared out weeks later.
The bag is a cute green tote bag I found at Goodwill once. The handle is long enough to wear over my shoulder comfortably, but short enough that if I need to carry it in one hand it works.
I've gone through every size of diaper bag from a regular purse to a back pack. I like this current size because I have room to put library books too.
And I've tried changing size of bag and contents based on the day and what we're doing and I end up driving her home with no pants on or lugging 5 diapers for a month after a big trip when we're only going on hour long trips. So now it's set up for a basic 2-4 hour trip to the Children's Museum near our house and I just replace things that end up in the laundry bag.