In Lady Grey's post, Roadblocks, she is going to be having a slave competition between her sub and some others.
In attempting to come up with some ideas under the parameters (it should be winnable and not cause any significant harm to a sub) here is one idea that I came up with.
Obstacle Course Race:
-Subs should have their hands locked behind their back.
-Subs should have an 18 or 24" spreader bar locked between their ankles.
-A modified ball parachute with a round tray mounted underneath it.
-A round object such as a ball or a hard boiled egg placed on the tray. The less perfectly round the object is, the easier it will be to keep it balanced.
-Difficulty could be increased by adding a butt plug.
Subs must maneuver a simple obstacle course while being timed without dropping the ball off the tray.
If the ball falls off the tray they must retrieve the ball and bring it back to the starting line where a Domme will deliver a hard swat to each butt cheek and return the ball to the tray while the timer continues to run. If the ball rolls somewhere that the sub cannot retrieve it they will be disqualified and punished (the idea of a Domme having to retrieve the ball for them just seems wrong).
The sub with the lowest time to complete the course wins.
Here are some shitty sketches I made in 3 minutes before I have to leave for work: