yeshivaguy > Rav Yisroel Reisman many years ago that Bein Hazmanim is to learn things that you normally wouldn’t learn in a yeshiva setting. For example, a short masechta.
A good idea. Or, as R Twersky suggests to learn physiology to appreciate miracles of Creation, but do it outside of yeshiva hours, you can do that too. Also, learn things that help understand Gemora but are not part of the curriculum: Nach, logic, Jewish and World history, chochmas yavanim 🙂