Business would be good if you intend on working as a project manager. Math would be another good minor and could even help you with your engineering classes. The following would also be possibilities to explore.
Management: Management can help a resume
Materials Science: It would augment your Civil Engineering knowledge
Computer Science: CS is the hot topic right now. A.I., Machine Learning, Augmented Reality, Virtual Reality. All of these would be valuable assets that would differentiate you from the crowd.
Enviromental Engineering: You will be amazed how much EPA stuff you will deal with as a Civil Engineer, this would augment your degree nicely as well.