If your story has an idle nobility class, their culture shouldn't just be different from the general population, it should be an over-the-top caricature of the common folks' culture. Whatever the population generally agrees is ideal, fair, admirable, or good, the nobility will take into stupid extremes.
Contrary to the beliefs of many, people are actually not at all happy when they're idle - a person with no assigned task or duty will go out of their way to come up with one. And all around the world, whenever there's been an upper class with nothing to do, they've started to compete with each other over stupid shit, but always stupid shit that the culture they live in considers positive qualities.
From the noblemen in Europe challenging each other to a possibly lethal duel over insulting someone's hat, to a Chinese noblewoman being moved to tears by the beauty of someone's calligraphy, bored elites everywhere have always wanted to outdo each other in their expressions of possessing all the noble traits that this culture in particular holds in value.
You can, and should, use this as a way to highlight what the actual values of this society is. In a setting where being religious is held as an admirable trait, there is nobility coming up with new ways to one-up each other in their expressions of worship. Society that values art and music will have them competing over who hires the most artists, and who employs the most talented musicians. Aggressive, war-like people will have fuels to the fucking death over a stupid hat.
Literally anything can be competed in, and bored people with far too much time and money in their hands will become competitive over the most ridiculous things. This isn't just an useful tool in worldbuilding, but also a fun one.
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