You can predict how long tasks/projects will take you (stopwatch and/or calendar time). Even if calibration doesn't generalize, it's potentially useful on its own there. And while you can't quite mass-produce questions/predictions, it's not such a hassle to rack up a lot if you do them in batches. Malcolm Ocean wrote about doing this with a spreadsheet, and I threw together an Android todo-with-predictions app for a similar self experiment.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Notes for future OT posters:
1. Please add the 'open_thread' tag.
2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)
3. Open Threads should be posted in Discussion, and not Main.
4. Open Threads should start on Monday, and end on Sunday.