Maybe no bag at all? One camera, one lens. If digital, maybe a spare memory card and backup battery. If film, maybe an extra roll or two of film in a pocket, cargo shorts pocket if summer, jacket pocket if cooler weather. Zippers on pockets are good. Travel light. Maybe slightly wider than normal lens, I happen to prefer a 35mm, but whatever you're comfortable with. Telephotos are for slackers looking for easy pictures with little effort or talent. If you don't want to interact with your subject, learn to prefocus and then practice shooting from the hip at that distance. If you can nail it without stopping down your lens, even better. Good luck. Be fair and honorable with your subjects. Homeless people should be off limits. Everybody does it and for no good reason beyond easy pictures. If you want to blend in, wear a dark shirt/coat (assuming your camera is black) to draw less attention to yourself.