Probably the most important thing is to study something you're interested in or are going to enjoy. I agree, learning stuff is never wasted and you can never tell what life will do with you - or how something you learned about, either through study or experience, will come in handy later.
As for recommendations - it's difficult because we are all different. If like organising things, or understanding a bit more about how organisations work well (or not), maybe one of the management disciplines is the way to go. It's what I did some years ago after starting out in ATC. I think it has served me well although I've never had any hankering to be a manager; I much prefer actually doing and achieving things! As the years have passed....and I have found myself more and more involved in aviation legislation (everything from drafting to interpreting and implementing).....I find I might have enjoyed doing law, although if you had suggested it to me when I was in my 20s I would have run a mile.
On a more general point, legislation is so ubiquitous a law degree could come in handy in whatever field you might find yourself.
If you're thinking about having a fallback if you lose your medical or whatever, and you think that you would want to stay in this business, then management with a specialism in some sort of safety would probably be useful. If you were to lose your licence and want to get away from aviation, then do something that will equip you to earn a crust. IT is an option, but things move on very quickly and skills you learn today will be of limited value professionally in ten years if you have done nothing related in the interim. If you're thinking that a qualification is useful at the end, don't just look at undergraduate/first degrees - in aviation/transport subjects your professional licence may well get you into a postgrad course.
But my real advice goes back to square one - if you're going to put effort and time into studying anything, make sure it's something you'll enjoy.