I think something along these lines has already been mentioned briefly in comments on the Roadblock post... but here goes.
A bucket of water.
A cup/glass with a "fill line" marked on it. A graduated measuring cup might work the best.
A spoon of some sorts (teaspoon, table spoon, etc., the larger the spoon the faster this will be done).
This could be done simultaneously or one sub at a time. The only difference would be if done simultaneously you can determine the winner by whoever finishes first and one at a time will require a time-keeping device.
Place the bucket of water and empty cup on the floor, 6' apart.
Restrain the sub's hands behind his back and put the spoon in his hand.
The sub must take water from the bucket using the spoon and transfer it one spoon full at a time in order to fill the cup to its fill line.
First one to fill the cup and deliver it to his Mistress wins.
-For added difficulty the handle of the spoon could be placed in the sub's mouth (instead of using his hands) but I have a feeling this would make this take much longer, although this might be the preferred method if you wanted to see the subs inching around the floor on their stomachs instead of moving around on their knees.
-Spilled water could be punished but I have a feeling trying to traverse through the spilled water is sort of a punishment in itself since it will make their skin stick to the floor.