Active learning is important in all modes of a HyFlex course. Many faculty and designers are familiar with active learning strategies in the classroom, and most of those may work well in a HyFlex course also. When students join the class live online (synchronous, for example with Zoom), some of the classroom approaches may still work with two types of participation happeening. See Derek Bruff's post explaining "Active Learning in Hybrid and Physically Distanced Classrooms" for some very good ideas about making this work. Online asynchronous students need different activities for active learning, and the faculty will have to be actively engaged in those, most likely, for the best outcomes. See this paper, Actively Engaging Students in Asynchronous Online Courses, from Shannon Riggs and Kathryn Linder of Oregon State University for some excellent strategies for active learning in the online asynchronous mode. You might be able to use some of these approaches for all students, as a form of out-of-class learning experience. That would support the development of learning community across all modes as well!