Ok well .. I'm not really thinking straight right now but Maybe lets say Twilight just remembers she has to go do an errand for somepony and Discord decides to follow her and he gets into doing chaotic things and almost gets caught or something. This could be at night, that way Luna has a better chance at getting involved.
I got it:
Twilight forgot that she had to go meet ummm one of the five ponies you pick, and he disguises himself as Spike, and hilarity ensues as he goes around being Spike, causing mischief next to Twilight and making ponies stare. Twilight is nervous the entire time, and right after she finishes meeting with that pony and THINKS she's home free, Luna pops up behind them, and is on to them ;P
Then moar hilarity when Luna get's dragged into the childish antics of the chaos god and purple pony .3.