I would always use hexaflexagons at the end of a term. I used Christmas Message as an Assembly more than once. The others fit into any of those times in the school year when the class needs something different – which includes the end of term, but also those occasions where you know in advance that their hearts are not up for the usual learning content.
The sort of ‘something different' I have in mind provides challenge but not demanding of much in the way of residual learning. Definitely not ‘in the exam’. You might view this as off the course and in a sense useless, or you might view this sort of material as the very point of the subject, something that it can be used for. Presented with sufficient care, a class can take the initiative and, by ‘taking possession’ of the work, provide its own enthusiasm. I was always surprised what succeeded and with whom – adjacent classes might have opposite reactions.