The unstructured blazer — softer shoulder, lighter fabric, no heavy lining — is built for exactly this. Worn open over a plain tee and trousers, it adds shape to an outfit without adding formality. Rolled sleeves push it further into casual territory.
Think of it as a layer, not a uniform. It's the piece that turns "I didn't try very hard" into "I know exactly what I'm doing" — which, most days, is the whole goal.