Sydney wore the cat ears that she had been provided, but other than that she just wore every day, regular old clothes.
She was in the habit of popping in and popping out kind of unceremoniously. She'd take some candy here, would blow out some candles there (she's in the same mindset as Jason on that one.) She mostly was racking her brain for what could be broken down after the event was over for resources; she understands the importance of keeping morale up, but she's ever pragmatic.
She wouldn't have lingered, or danced, or really have done anything remotely fun unless someone asked or dragged her into doing it.