We've got a medium sized backpack from my1styears, it's personalised with his name. It's a decent size to keep his things as well as his inhaler, snacks, etc x
I use his nursery backpack or pack his stuff in a newborn bag if we go out with his sister too
I used a regular back pack with his things in we are still using the pram though
Backpack. Spare clothes, nappies, wipes, few small toys, snacks and water bottle x
We just use a normal backpack x