There are lots of ways in which you can randomise self tasking and this is a good one. I recently experimented with my sub on a method of doing bathroom control which did not really work. I told her that, at work, she had to pick one toilet cubicle and could only use that cubicle all day. This meant that potentially she may not have been able to go to the toilet when she wanted to because someone else was using the cubicle (even though other cubicles were free, she could not use them unless it was an emergency).

It did not quite work. The limit was not really strict enough - not enough people using the cubicle - but it was worth a try and now I know to think of something harder next time.

You could do this sort of thing on real sports as well - set an outcome and try to predict which football team or tennis player will win. If you set some task that must be done during the match as well (i.e. unable to go to the toilet until it is over, must masturbate but not cum until it is over, have legs tied together or be gagged etc) then it makes it more interesting. Especially if the tennis match overruns as long as the recently famous one at Wimbledon (12 hours in the end, wasn't it? ). I wouldn't suggest this sort of thing for tennis or cricket...

Dice can also be useful for randomising things. You can have a list of tasks numbered and roll a dice each time to find out what you get. If you have polyhedrom dice you can get up to 100 or more tasks in that list but even with a normal 6 sided die you can get a few options - roll it twice for 2 - 12, three times for 3 - 18 and so on. Just be aware that this method tends to make the middle numbers (6 - 7 and 8-10) more likely to happen... You could also have a system where you have 1-3 means 'something good' and 4-6 means 'something bad'.