Wow, this started a great conversation. Using a trunk bag sounds like a good idea, although I do carry a laptop on occasion, so I'd still need a pannier for that. I haven't seen a trunk bag that is sized for a laptop.

Someone mentioned a Thermos for soup and liquids. I think that's exactly what I need to buy. I've been using kitchen storage containers, but find that they can leak slightly when being jostled in a pannier.