I use a Camelbak Blowfish these days for my commute. But I have a paddling "burrito" shaped bag that bungies very nicely onto my lightweight rack (Tubus Vega) when I need it.

Lately I've been doing a Saturday morning run to the office with clean work clothes for the week (from there I head out for other errands or just a Sat afternoon ride). I do use the rack bag and an older North Face rucksack for that. With pants, towels, and unmentionables in for the week, I just use the Blowfish Mon-Fri for a clean shirt, bit of food, and to bring back the dirties every day. Keeps my daily commute light and fast (relatively speaking ).