Depending on how much you need to carry...I use the patagonia atom sling bag. Only 8L, but the shape means no sweaty back, the stabilizer strap means it stays put, and it holds the basics. It also has compression straps so you can strap down a jacket or something on the outside. I think they make a larger one as well. Smaller bags make you decide what you really need to carry, which can be a good thing. I also toss in a nylon backpack that folds into a small zipper compartment - in a pinch, I can layer it over the sling bag if I have something unexpected to carry.