I would think about it in terms of what you like, in one of two ways:
-Is there another, very different subject you really love but don't want to major in? This could be a good minor if you want to keep learning about the topic. It could also make you seem well rounded, as Dave says.
-Or, is there a complementary subject that you want to specialize in to help target your computer science career? In other words, what do you want to use your CS degree for? Do you expect that you'll need/want a lot of background knowledge in, say, math or statistics? Or maybe you want to use your coding skills to work in a more science-based field, and might want a biology minor.
In my experience, a minor has been useful for 'putting me over the edge,' so to speak. It's probably not going to get you a job if your coding skills aren't as good as another applicant's, but if you two are equally good and you have a minor in something really relevant to the company (e.g., biology for a medical/science company), it might break the tie in your favor. A good example of this is positions where Spanish skills are preferred but not required - if 2 applicants are equally good on the required skills but one knows Spanish and the other doesn't, the one who knows Spanish will probably get the job.