I want to get a bag to carry a modest amount of stuff.
How much stuff? Like: keys, money, phone, jacket, maybe some rain pants, some food, a couple of tubes, a small toolkit. Sort of enough for a day's unsupported ride in variable weather.
Ideally the bag would be: waterproof enough for a rainy day, attractive on a variety of bikes from classic steel to carbon, easily moved between said bikes, as light and unfussy as possible.
What do you use? Handlebar bag? Small saddlebag? Big saddlebag? A seatpost-mounted rack with a rack bag? Material? A specific brand to suggest?
I know nothing about rando stuff. I've had full-blown panniers, but am thinking of something a good deal smaller and lighter.