It all depends on why you are studying math.
Linear programming is a relatively narrow branch of applied mathematics. If you are looking for what may be highly useful in certain very specific jobs, linear programming is such a skill. But frankly, if you are interested in math to help get a job with high pay, I might suggest either numerical methods or discrete mathematics because there are more jobs that require knowledge in those areas.
Calculus on the other hand is a very broad field that is a prerequisite for a whole bunch of other math (like statistics) and for other disciplines (like physics, engineering, economics). But absolutely no one will give you a job just because you know calculus: tens of millions of people know it: it has no scarcity value.
If you are interested in math as pure intellectual exploration, a fun course is abstract algebra. I warn you, however, that it is hard.