One of the best things I've done as I've fumbled through learning how to teach, has been to routinely ask for student opinion. I've done this both anonymously and not, but the answers are always enlightening. I administer a mid-semester evaluation of the class to see how things are going, but I also find that asking students about what topic they liked the best over the last X time period can yield surprising results. On my last midterm exam I used this as a bonus question and found the the answers converged roughly on three topics. Two of them were side things I tossed in to the lecture because I thought the students would find it interesting.
Based on the feedback I may find ways to expand a bit on those topics in future years, but more than that, it gives me a bit more information on the types of subjects that engage the students. Biology is weird and fascinating and I like to make that clear. Sometimes I can grab the classes interest and sometimes they seem to glaze over. Asking them after the fact provides a lot of insight into what engaged them either in class or when they were studying later. Sometimes it's surprising what stood out to them.