Although MIS is somewhat of a business related degree.... I would recommend accounting, whether you end up sitting for the CPA exam or not. I work in IT and a good understanding of accounting (and business) is valued. I wouldn't recommend double majoring in business with the MIS as many of the same courses would be included.
Other options (dependent upon your interests and the majors available at your school) include: Biology Economics Public Health Statistics
It really depends what you want to do with the credential (this is not a flippant answer, but merely one intended to impel self-investigation). Do you want a career in IT management? Provided you have several years experience, it might be advantageous to get a dual-degree in IT management. Do you want a career in accounting, or as a management consultant? You might want to consider a degree in accounting, and (of course) a CPA would be an advantage. If, however, you wanted opportunities in the financial industry, one would desire a dual degree in finance. IT is always a safe bet to accompany any business-related major. As I have seen, a credential in MIS (coupled with the appropriate experience) is of sufficient weight (depending upon the reputation of the university) that you should not want for prospects.
