I was wearing a jacket, so some of that stuff fit in my jacket when I was carrying around this travel bag. Also, I'd sometimes carry around a larger tote bag instead, or I'd carry around a plastic shopping bag in my pocket so I could put stuff it in as needed.

Patrick is right, many people in Europe (especially here in London I've noticed) do travel to/from work with a backpack/rucksack/daypack (are these all the same thing?)

I guess in many cases this holds the whatever things people need for going to the gym or doing whatever sport etc at lunch or after work. I'm sure people use them for all sorts of things but wearing one will not be the obvious giveaway that you are a tourist.