I've been toying with an idea for what I'm presently calling the Periodic Assignment module. The basic idea is that a user should be able to subscribe to get intermittent assignments from a Drupal site. The user would then complete the assignment and the site would log the completion of the assignment. For instance, given a set of different exercises, I could have the site email me once an hour with a random exercise to do. Or for people with a practice of journaling, the site could have a list of topics on which they could write on and once a day the site would send them one of those topics as a prompt to journal.
It's still very much conceptual at the moment, but the idea is solidifying as I mull it over. If you're interested, what documentation/planning I've done so far is at the link above.