I need to get riding again, and the new job has showers, lockers, and bike storage. I need to bring back and forth my laptop, lunch, the essentials for clothes change, and stuff I haven't thought of. I don't have a bike that with rack eyelets, so I'm curious your thoughts on backpag vs rack. I'm in SoCal, so weather is not an issue.
Have you considered a saddlebag? My Zimbale 18 liter bag (similar to a large Carradice) holds my lunch and scrubs and underclothes for work each day, plus my tools, spare tube, wallet, phone etc. In conjunction with a Carradice quick release saddlebag support that clamps under the seat rails, it just takes about three seconds to remove the bag and sling it over my shoulder.
Backpacks are generally unpleasant to cycle with.