Hey,
I always ponder what would look good on my CV. I ended up doing a distance learning physics course last year (As you can never have too many qualifications as they say!
). I was in the Air Cadets for a few years too, I don't know if you are, your still old enough to join. It will put you in good sted.
I don't know if you've started driving either but i've just started a course on advanced driving with the Institute of Advanced Motorists (IAM), that should hopefully look good!
I've also been thinking about doing something like a first-aid course. I always like adding another bullet point to my CV! Aviation is my main passion but I'm trying to show that I have a much broader range of interests. (I.e. driving, Mountain Biking, Photography, Engineering etc) I'm told airlines enjoy this as it will give you something else to talk about to the chap sitting next to you on the flight deck on an 8 hour flight!