I've been to Mumbai, though for a few days only and on a conference, so didn't have much time to explore. Very large and very busy city. You will have many opportunities to photograph street life - it is very colourful. Go to any food market! The train station looks interesting: very ornamental, almost Gothic, Victorian building. The oval-shaped Mahim Bay (city centre) looks interesting in the evening or at night. If you manage to find a high spot in one of the tall buildings by the sea (a hotel or a restaurant), there are good views at night. The locals called the bay 'pearl necklace' because of the shape the street lights form at night.