I'm taking a two year course on philosophy and ethics. At the moment, we're studying Kantian ethics. I understand Kant's theory of "act by a maxim that you would want universalized" and the three steps to see whether a maxim can become a categorical imperative, such as universalization, seeing humans as an ends mean and kingdom of means.
I am currently stuck at figuring out what maxims could be universalized apart from not lying and 'prisoners of war should not be killed'. What are other examples of maxims that could be universalized?