I have a Revelate Tangle bag that swaps between my Salsa Vaya and a couple of other bikes. It's a really nice quality bag. Big enough to hold a bunch of stuff, but small enough to still allow room for water bottles and frame pumps, and not get in the way of your legs. It even has a hole and routing for a hydration bladder and tube.

Here's a pic of the bag on my old steel Schwinn: