+1 on the Osprey daytool packs. I have an Osprey Focus (slightly larger than the Flux) and I think it's the best daypack I have ever owned. I bought it for a hiking pack that I could take travelling on business trips and safely carry a laptop inside. It's great for bike commuting, I really don't notice that the pack is there when I'm riding. I put a Camelbak unbottle in the hydration/laptop sleeve. I have noticed the top of the pack sometimes bumps against the back of my helmet when I ride on the drops, but not enough to be annoying.