I've never understood why cyclist use backpacks, unless your ride is so short that the issue of attaching a pannier or trunk bag to the bike takes up too much time. The added sweat and weight/strain is a no go for me and my half hour ride.

That said, I have a pannier but don't use it much for commutes. I use it a lot for shopping or errand running using the bike, but it's too big for commuting. A trunk bag that I strap to my rear rack does the trick for me. I fit a lot into it, and it has several zippered compartments for necessities.

To be candid, my trunk bag is really my purse. My wallet and keys go in one compartment, my tools and patch kit in another, and my rain cape, lunch, and change of shirt in the main compartment. It's expandable, which is great since sometimes I need to lug my coat or gloves/hat with me home, since it warms up on many days.

Love my trunk bag.