My day-trip bag is a
canvas sling bag, which holds my e-book reader, my water bottle, and anything else I might need while out and about (sunglasses, portable battery, spare USB-C cable, painkillers, etc).
My Kobo Clara HD used to fit in it with room to spare, but my new Sage is a bit of a tight fit at the moment due to the makeshift sleeve I've got on it from a 10" tablet. I might look into that Etsy shop that someone else posted for a form-fitting felt sleeve.