We have a normal back pack with some snacks (cereal bar, chocolate, fruit pouches), and we always have water bottles filled in fridge, and a 2 litre bottle on the side. The bag itself is what we use as a change bag and contains stuff we would need if we had to leave within minutes (E.g. for a medical emergency). Change of nappies, wipes, pain relief, coins, hand sanitizer, carrier bag, chewing gum, pen and paper, book, small roll of medical tape, wipes, magazine with stickers, tissues, eye mask and ear plugs (we have enough overnight stays it makes it normal for us), my meds. I keep my sons tablet and my kindle charged and stored with the bag and our coats by the door. Not for ease of grabbing purposes, more so I remember where they are.
We usually go out in the car or with the buggy and they both have blankets (plus food, water, and walking boots/hats in the car).