I just had a horribly hilarious idea.
At some point, Remus and Sirius get to drinking, and reminiscing about the good old days at Hogwarts. They get to talking, and make a list of "rules" about various things that would be hilarious.
Now the next day, Sirius gets to thinking. What would be an epic prank? Get Harry to swear an oath to break every one of those rules. If things are supposed to get worse, he's not allowe to tell anyone about the list.
He would have to trick Harry, but I doubt Sirius is going to worry about that too much, since it's for Harry's own good, and think of the hilarity that would ensue.
Obviously, parts of the list here would have to be excluded, since crossover rules and similar rules wouldn't be applicable, but that still leaves a good amount of hilarity.