It depends on where your interests lie. Math is all around us and is involved in most everything we do, so any subject will work.
When was the last time something made you stop and think "that's cool--how does it work?" What was it and what about it caught your attention?
Just live your life and something will eventually catch your eye. Every "huh, that's interesting" moment is an opportunity to dissect it and learn the math that's involved.