TL;DR: The Retrospective Exercises Repository
How to prevent retrospective boredom? One way to achieve that is never to repeat the same combination of retrospective exercises twice.
Avoiding repetitions might sound like much work for a single team. However, if your product delivery organization comprises of more than one Scrum team, I can highly recommend creating a retrospective exercises repository as it improves the quality of the retrospectives and saves much time if you share the retrospective exercises with your fellow scrum masters.
Learn how to build such a retrospective exercises repository.