Could be fun
Set strict 2 hour bathroom breaks. Then gradually increase the amount of water you order them to drink eventually they will have to chose between. Either failing and using the bathroom or pissing themselves. If they have no limits then you can take this further.
By ordering them to piss themselves then use the piss soaked underwear as a gag (hold it in with duck tape) how long the gag has to stay there is up to you a minute an hour overnight all weekend it's up to you. Of course if you go for a long one then I suggest ordering them to provide a freshly soaked pair after the taste has gone lol
let us know how you get on.
I also quite like diary writing although this will depend how verbal/literate your sub is if you get a good one a journal can be an interesting window into thier thoughts or emotional responses following certain tasks