I rock something similar to what Snuts showed (minus teh foldout panniers) and i include a freezer doogie like what rumrunn6 showed.

i typically have some nuts in a small snapseal container, a greek yogurt, an apple, a Premier Nutrition protein shake, and a small container of leftover whatever i can find - soup, casserole etc.

This is a great question and it took me several months to figure out how much to bring, how to keep it cool enough to avoid spilling, and how to eat (several small meals throughout the day - almost like repeated snacking)

Packing a lunch for a cycle commuter is much different than packing a lunch for a car commuter in my experience.