If you won't ever have to carry it and it will simply stay in the vehicle, sport duffle bags are ubiquitous enough to be totally camouflaged in an urban situation. My choice for an "in the car" container is a $4 lidded 18-gallon Rubbermaid tub from Wally World. Mine holds spare warm clothes, cheap vinyl poncho, first aid kit, a few liters of water, Power Bars & snacky foods, butane BBQ lighter & candles, duct tape, cheap tools like pliers/screwdrivers/crescent wrench, SAK, and a Cold Steel Bushman. And there's room for more stuff if need be.
If you have to carry it or it will be in outdoor environments, then Scott's recommendation for a backpack (or a bookbag depending on the amount of stuff it has to hold) is on the mark. IMHO getting a pack in flat & neutral colors like mud brown, medium gray, brick red, or dull blue makes the pack less conspicuous, even in an urban crowd, than if it is made with light/bright/neon colors. While I advocate getting a hydration pack for ease of always having water with you, in a SHTF situation you may want to retract the tubing inside the pack to reduce the "tactical" profile of the pack.