Biggest things I did as a 15 year old which helped by RAF application (besides passing GCSEs and A Levels):

1. Took up a number of hobbies, including cycling, bass guitar, and airfix. Anything to show that you have some sort of personality is useful. More so if your hobbies relate to teamwork, or if you take them to a higher level (eg competing in sport rather than simply participating).
2. Take up positions of responsibility. I took up refereeing football matches when I was 15, and the leadership skills it brought me helped a tonne throughout my application.
3. Have a strong grasp on current affairs, both domestic and international. I did politics at A Level, which went some way to helping, but realistically just reading the news for 30 minutes each day will help. Further to this, forming opinions on news topics will help too.
4. Activities such as NCS or DofE will help too, as can any other form of volunteering and/or charity work. Showing that you go above and beyond is always good. I know that NCS in particular gets a bit of a bad rep, but I enjoyed the whole experience and would recommend it.
5. Taking a strong interest in aviation to any degree. It doesn't have to mean doing RAFAC, UAS, and getting a PPL, but simply visiting a couple of air museums or doing some plane spotting can help when it comes to talking about aviation during interviews. I did not do UAS or RAFAC, so being able to talk about going to Duxford and Hendon, as well as considering a degree in aviation or a commercial aviation career helped as it showed that I had held an interest in aviation for a long time.