Well done for getting through to the next stage mate. I'd say dress smartly, and be the guy who actively speaks to everyone without forming a group, and sticking with the same people. Make it known that you can confidently communicate with different people - as people are watching everything you do...
For the interview, think about some examples of times you've shown leadership, motivation, team work, dealt with conflict etc... and try and practise some answers - detail is important, because they may ask you follow up questions. Too much detail is better than too little... make eye contact, and speak confidently and at a good speed, and always relate any answer you give to how it will ensure you can be the best within easyJet's culture.
For the group task, try and let everyone engage, bring in those that are quieter, and try and manage the task, whilst still allowing others to suggest and make inputs too. If you aren't managing the time, try and take charge of another element... and always appear to be trying to involve those that aren't saying much. At the same time, if anyone is trying to over-control things, perhaps say, "right, before we move on, why don't we make sure everyone is happy first".
Have fun too, it's a nice day, and I definitely found it more enjoyable than the PILAPT and Maths. Hope it goes well bud!