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I have one of these musettes from Waterfield, the canvas version in blue. It's retardedly expensive for such a simple bag but it was a gift and I really do enjoy it. It's the highest quality bag of any type I've owned and the most presentable. Easily fits a 14 inch Macbook (but you could probably cram a 15), sketchbook, chargers, over ear headphones, snacks and other small items. The engineering concept of Keep It Simple Stupid applies just as well to product design of mundane consumer goods.
The other option being I wear a cheap lil adidas waistpack on some of my longer roadie rides to carry spares, snacks, external battery or even a third bottle. Around town I can pick up a sandwich from the deli or something in put it in that, carry my wallet, grab shit from the pharmacy, etc and wear it as a crossbody.