For me, this is a leisure issue, and not a commuter one, but most bike commuters tend to carry stuff with them, which I'm lucky to be able to avoid. So I'm hoping someone else out there will get some mileage out of this. In my case, there's a great park a few miles from my house, which is too short to drive, but sometimes I just want to read a book in the sun. Without racks, panniers, or a backpack, that's been easier said than done.
I use sea bags while kayaking. They're waterproof enough to put a digital SLR camera with an expensive lens in, tie this to the hull, and set it in the lake. ( You can't tow it because of the drag, but the bag holds the water out. ) Boat people love rope, and so these things have D rings on the ends. You can clip this to your belt with a carabiner. I'm finding four liters to be a great bag size.
This holds a book and a small snack, or a bit of other gear. There's no sweaty back, no gear for the bike if you prefer a "naked" cycle ( but not rider! ), doesn't hit the wheel, and attaches near your center of balance, so it's pretty comfortable. And the total cost is around $15.
I don't have a picture of me riding with one, but hopefully that pic is helpful. Also, while this is a cromulent way to carry lunch, or a book, I don't think it would work for bigger loads ( clothes + shoes, or a laptop ), because the bag is going to swing around and get in the way. Still, victory is mine (!), and I wanted to share what's been working for me...