My professional dog walker bag contains:-
Dog first aid kit, the one I was given as part of the dog first aid course I took.
Poo bags.
Treats.
A ball.
Hand sanitizer gel.
Collapsible bowl.
My water bottle.
The dog's water bottle.
Insect repellent for me.
Tick tool for both me and dog, in case the insect repellent doesn't work.
Ceterazine (best known brand is Pireteze but Tesco Own are fine) tablets, for me in case of horsefly bites. Amazing! Also I have heard that they are very effective for snakebites for dogs but have no personal experience.
Loperamide tablets for me, best known brand is Imodium. Getting the runs in the middle of nowhere is something I never, ever wish to experience ever again...
A spare slip leash in case of broken harness or collar.
It also contains of course my phone, van keys, house keys, purse, all the clients keys, and anything that I would be really inconvenienced by losing if some toerag did a smash and grab on the van.