Two science majors, one of which exposed me to a good deal of biochemistry. Having the basics of biochem was very helpful for med school. Other than that, a lot of pre-reqs for my majors that were introductory classes for other fields (molecular genetics, microbiology, immunology, etc) were extremely helpful. I tried to learn all the material to the best of my ability, even when I could have coasted to easy A's in some of these classes, and I have been surprised at how much it helped in the non-organ system intro blocks of med school. There was almost nothing I was seeing for the first time, and the solid foundation I built in undergrad paid off. I had some free space my senior year, so I filled it with introductory pharm classes for kicks, and the fundamentals I picked up there are also quite useful.
That said, I didn't take those classes as a way to prep for medical school, I just followed my interests. I would recommend that people thinking about which major to follow do the same. Just don't discount any of the science material you have the opportunity to learn at a leisurely undergrad pace. It may come in handy later.
