Over at Infin there’s a nice post on recording Org mode activity. What that means is logging all the little administrative steps you take with your agenda such as marking an item done, changing its state, adding a note, and so on.
If you have a need to audit your actions—exactly when did I change the state of that task to WAITING—the functions in this post could be a big help. There’s also some code to query your agenda for things like “Show me all the activity in the last two days.” Not everyone will have a need for this type of thing but if you do, the post may give you some ideas and a starting point.