Hehehe-- most folk just want a DATE with no baggage ;)
Gimme a small pack with a couple water bottle pockets. Other than basic clothing like shirt, pants, shoes, wind shirt and a hat, all I would need would be minimal toiletries, a really Spartan change of clothing: spare briefs, socks, running shorts and tee shirt, mostly to wear while cleaning the other stuff. I would have plenty of room left for a book, notebook, cell phone and camera. Most of the clothing would be to avoid buying new stuff all the time, which could be done, but would be wasteful of time and resources. Something like a Patagonia Travel Pack or an REI Flash 30 would be more than enough.
We traveled in Europe for three weeks in the dead of winter with simple convertible carry-on/backpacks and that was in 1986. With modern synthetics and personal electronics, I could go indefinitely with less weight and space. It is a lot easier to travel light in cities-- no food prep, sleeping gear or shelter. We went by train and a one-bag system is so simple and easy. Seeing someone in the airport with a mass of big suitcases is just like seeing someone on the trail with a monster pack.