If you really don't intend to use focal length greater than 50mm and heavier cameras than film 135, I think most anything would do. I sometime use a very light no-name no-quality-whatsoever tripod of a friend of mine that, at the end of the day, does its job adequately. Sometimes good enough is good enough. That's mainly for wide-angle nocturne photography. The "problem" is that once you begin going around with a tripod, taking time to find places etc. you end up using also some tele and making several shots, with different focal length, from the same location.
From my experience, when using a tripod in town the real problems are vibration induced by the buses and lorries, and maybe the underground. Those are frequent sources of "hidden" vibration that can affect image quality, with the blame later put on the tripod.
A very convenient thing Gitzo tripod have is the hook on the base of the central column. You can carry a light tripod but, when you hang the photographic bag to its hook, the tripod acquires a bigger mass and becomes much more solid. Useful in case of wind, and with tele lenses.
I understand you don't want a Gitzo, maybe some other make has this hook or maybe there might be a way to adapt one to your tripod.
I used to go around with my Manfrotto Triman #028, a beast of more than 5 kg with head #029, basically thought for studio work, now I do it only on some rare occasions. The idea behind this conduct was that I could use my 500mm or 300mm. Going around with heavy tripod and vast choice of lenses (from 24mm to 500mm) is something that I feel less and less inclined to do now. I decided my back is my friend and, after all, I did not use those lenses much. And so I discovered that most any light tripod is good enough for normal to wide-angle lenses.