For an Officer role you want things that give you responsibility. Charity shops/retail can give you cash handling, dealing with customers, maybe stock control, key handling. With sports clubs or any uni societies, it's not just participating, you want to look at the committee positions like secretary and treasurer etc. You may be able to get involved with youth groups without DBS initially if there are sufficient qualified other staff; do you have anywhere doing children's activities for the holidays? Local church or community centre doing fund-raising? Could you organise a charity fund-raising event, get some mates involved, maybe for a half-term if you need a bit more time?
Bear in mind that if you apply too soon before the end of uni, and have the GCSE and A level results, you are eligible for the role and if it's open, they will start the selection process. You have some choice of dates, but it will be picking from what they offer; you can't be deferring for weeks if it clashes with end-of-year projects and exams. They have to time things to try and get sufficient candidates for an IOT. Look at when you can afford to be taking time off.
There's plenty of info on here re interviews, and you'll get more from the P2 presentation once you've applied. Quite simply, the first half of the interview is about you, education, hobbies, sports. Second half is about your chosen role and the RAF.