My original exploration of the business aspects of photography was focussed on stock photography. In retrospect, the passive nature of the stock photography income was the most attractive feature: you shoot, process, keyword and upload a photo once, and after that, it just keeps selling (that is the theory, anyway). Also, there is an interesting aspect of trying to figure out what type of image would be attractive to potential buyers. However, because the range of buyers is incredibly wide, what bubbles up to the top are rather generic and not particularly interesting, from my personal perspective, images.
Eventually, my interests shifted to sports and events photography. In part, this is because this niche provides more opportunities for interacting with people, who are involved in the photo shoots. From the business perspective also, the type of photography that delivers both a product (images for sale) and a service (event coverage) has been more successful (at least in my particular case).