Minors themselves are rarely "in demand." Really all a minor is, is a formal recognition that you have taken more than an introductory course in a particular field. They can help you in job searches sometimes in that they provide evidence and formal training in a field that might be applicable to the postions, where a formal degree in that area is not necessary for a position.
As to what pairs up well with physics, that's up to you. If you were interested in geophysics and your school doesn't haven a geophysics-specific option, a minor in geology might help you get into that field. A minor in chemistry might be beneficial if you were interested in material science.
The down side of committing to a minor is that it tends to restrict your freedom to sample other fields.