I am very unoriginal. I have the Harbor Freight 15” foldable / collapsible tool bag that I have yet to install on the back of my bike. For an around town runabout bike it stays zip tied to the rear luggage rack and provides a rather large amount of storage space when you want to pack items such as from the grocery store or wherever and cart them back home without having to resort to taking a backpack with you. And when not being used it folds completely flat against the rack.
You get them for $8 or $9 from HF.