This may be a good one if you can remember to keep it up. Got the idea from a set of photographs I saw. It was of a family of four, taken every single year at the same day over a period of something like 14 years. It was naturally pretty amazing and interesting to see how both parents and children changed over the years.
So… what could you do with that? If you blog about health of weight loss, this is a good one to do. A picture every year of yourself or your students if you coach people.
In business, you may want to share about how much your business progressed, what are the high points and low points.
In just about any niche market you could do like a yearly round up. Remember, it need not be your experiences. It could be your clients/customers.