It's very difficult to generalize about post-graduate requirements because of the many options available to Biology majors. For students planning to go on to graduate school, be aware that many programs require 2 semesters of Physics, as do medical and many other professional schools. Look into the requirements and recommendations of the sorts of post-baccalaureate programs you're considering.
Depending on your interests, courses in economics and statistics may also be helpful. With the ever-increasing flood of opportunities in bioinformatics, some experience with computer science is becoming increasingly advantageous.