The trick I've learned from one of my aunts is to add a little bit (half a teaspoon for one of those paper boxes of mac, not much more) of mustard to it, somehow makes the cheese taste sharper as long as you don't overdo it and make the mustard flavor too obvious. I've also found adding a fairly significant amount of black pepper does good things to it, along with a little extra cheese melted in.

I also put a bunch of spinach and a bit of salsa in mine, but that admittedly dramatically changes the flavor instead of just enhancing it, I just happen to like the result after some experimenting