Mumbai has a number of historic markets that offer eclectic and economic shopping, and some colorful photography options.
Spread across the city, some of these are speciality markets (clothes, antiques, spices, flowers), some have a bit of everything, a few are enclosed, and others set up on the sidewalk. Some are organized and others are not.
If you are looking for affordable fashion, try Lokhandwala Market in Lokhandwala, Linking Road in Bandra, and Colaba Causeway; for antique goods, try Chor Bazaar, and so on.
You can also sign up for one of the many market walks on offer.