A hard one to answer.
Firstly don't worry about making your application "stand out". This is not a civilian job application, at each stage, if you pass you will move forward with every one else. Only your final two selection stages, POC and AIB, are scored. To be selected for YO training you will need to achieve high marks in both.
Do not worry about not having a degree. If you have a first from Cambridge, but spent three years playing Xbox in your spare time you are not passing AIB.
Just get yourself out in the world - sport, clubs, part-time jobs etc. Do some voluntary work if you can fit it in. Anything that gets you interacting with as many people as possible.
The thing to remember, is this is not a tick box exercise. At AIB they will want to know what you gained from these activities, how they made you grow as a person and potential leader.
intellectual maturity, judgment and personality will be scrutinised at AIB. The board isn't looking for the finished article; just evidence that the basic ingredients are present.
Think hard about your motivations for wanting to be a RM officer and be able to convey these in a succinct manner
And last - be yourself! Yes I know that old chestnut, but at the end of the day you are either the right personality for the job, or you are not. Better to find out during the selection process than 14 months into YO training.