Gretchen Rubin, who wrote “The Happiness Project” and more recently, a book about everyday habits called “Better Than Before“, pointed out that people are most satisfied with what they do if their activities are new and challenging, and also if the activities support an environment of growth. This means that a fulfilling pursuit has a cumulative effect, which we can see and enjoy over time. By the way, this does not mean accumulation of something material. The effect can be in the process itself, as in writing calligraphy with water on the sidewalk, for example.
Examples of activities that cultivate growth are gardening, collecting (stamps, coins, etc.) and raising children. The latter is particularly rewarding, since, as any parent would tell, kids grow very fast, both physically and psychologically.
I think that, by extension, photographing children (following the development of the same child over time) is an excellent long-time project. It develops the photography technique, provides a way to connect with the child and creates a cumulative record of memories that the child would be able to access later in life. Incidentally, having this visual reference is tremendously important for forming a post-factum impression of one’s childhood, since visual images can shape our memories.