Queency - I didn't make it to the assessment day, so I'm afraid that I can't tell you first hand.
Melbourne just has an open day (150 people), and I haven't yet heard from anyone who got through to the assessment day!
But from what I've read, the questions differ each time, and between cities. I guess they're not going to use the same questions every time so that people can't 'study' for them.
But they're looking for team work, how you present yourself, control of the English language, etc. I guess there is no "right" or "wrong" answer to the question, you just need to work well with the others on your table.
If you have trouble coming up with your own idea, perhaps just say after someone else speaks "Yes, that's a good idea, I really like how you've though about _______". Just so that it seems like you're paying attention to what others are saying any things
But anyway, I've never been to an airline assessment, I've only just started applying to airlines, so I'm new to this too!
Good luck though!