My first recommendation would be broadening your scope of interest and taking other kinds of classes (assuming you can get a good grade in them). For example, I plan on taking some language classes, some math classes (linear alg/diff. equations), maybe some programming classes, and a psychology minor.
If you're really set on biology electives, I hear that neuroscience and immunology are some of the courses that med students struggle the most with, since most lack a lot of exposure to these two subjects and there are a lot of terms to become familiar with. Taking them would probably make your med school experience smoother.
I've heard a lot of mixed opinions on histology (just because many dislike the undergrad course), but I hear it can also be helpful to have previous exposure on this once you're in med school.