Look through the syllabus subjects or something you’ve done, as you mention.
Go for something you feel interested in and maybe even have some photos/video.
Which idea did you get first? That is most probably the best one to do as it is the most memorable to you.
My biggest tip… Speak slowly and you will use up the time quicker than you think!!
Why not combine them and make it
My firsts in the air cadets,
Then you can add in your first night, your first flight, your first camp and maybe your first course
That way you can share the experience of what you did and how you felt to younger newer cadets
When I set these for cadets (often for senior cadet selection) I usually choose an operation in the past e.g. Op Chariot or Op Black Buck (depending if you want to stick with RAF or not). Plenty to talk about and you can discuss whether we could do it today (or would want or need to).