I was a biology major in undergrad, but there were quite a few classes I hadn't taken upon starting med school, such as human gross anatomy, physiology, biochemistry, or immunology. I did absolutely find (actually very well) in med school without having previously seen any of those subjects.
The best thing, in my opinion, is to get some clinical experience. I worked in a hospital before med school (as a full-time employee with benefits, not shadowing or volunteering) and it was the absolute best thing I could have done for myself. I am far ahead of many of my classmates in the clinical aspects. Some of them are so incredibly clueless as to what goes on in an office setting or in a hospital. Sometimes it's almost like they have never stepped foot in those places.
So, if you can, get some clinical experience and work in a hospital or something. It will serve you well and, in my opinion, is actually better than taking a class that you'll just take over again anyways. It helps to have an idea of what to expect when you start your clerkships, and honestly, it gives you a little bit of "real world" outlook; that is something you can't get anywhere else. I think it helps make you well-rounded. So many of my classmates have never even had a job before med school, and they're so whiney and entitled it's crazy. Ok so my rant is over.