Either dinner leftovers in tupperware from the night before, or sandwich is plastic bags, or a random selection of things I grab real quick before I run out the door (apples/soup/yogurt/hummus/etc). If I have have more than a single ziplock bag or tupperware container, everything goes in a grocery store plastic bag, and then into my handlebar bag. 12 mile commute each way.

Coolers are for sissies! Of course...my stuff goes into a fridge at work when I get there...if food is sitting out for more than 4-5 hours you might want to think about worrying depending on what food it is.