If you're physically or chemically-inclined, I absolutely loved pchem II/QM. If properly designed, it will function to explain the origins of many things taught in undergraduate physics and chemistry that we are just taught to accept. Many lectures included 'ah-ha!' moments, where the underpinnings of things you had learned as a freshman are revealed and derived.
As far as biology/medically-related classes go, any class that allowed me some sort of freedom and that was not lecture-based. I took a biochemistry II course that was taught almost entirely from the literature, and an advanced cell/molec bio class whose end goal was to write a practice R03 grant, with a project entirely conceptualized by individual students. These classes were fun and taught me critical thinking skills well outside the realm of possibility for typical lecture courses.
Outside of the sciences, I really enjoyed my intro philosophy lecture - the problem of free will and determinism is an interesting thing to think about; this course also touched on ethics, which is obviously important for future physicians.