I've finally decided to give a protagonist a happy ending, but what is there to avoid? What makes a happy end too cheesy? For example, if the story is about a anti-hero, who destroys his enemies, what can I avoid when writing the happy ending? What makes it too cliché?
I would turn the question around: What makes a happy ending work?
I think it's this:
The character has earned the happy ending. The happy ending comes about as a direct result of the main character's actions. At the very least, the happy ending comes about as a result of the main character realizing something (as in The Wizard of Oz, where Dorothy realizes that happiness is home and family).
The main character had to struggle for it. The happy ending did not come easily. To bring about the happy ending, the main character must do something difficult, or come to some difficult realization, or sacrifice something important.
So avoid having something or someone other than main character being agent of the happy ending. Avoid getting to the happy ending without a struggle.
A non-cheesy happy ending is generally a mixed bag; the hero may not get everything they wanted, but they did get what was most important to them.
For example, they didn't get the treasure, but they found true love, and nobody else got the treasure.
Or they succeeded in sending the CEO to prison, but they lost their job, or also got convicted and had to serve a year's probation. Or in achieving victory they lost someone they love.
Give them what they want most, but make it come at a cost.A lot of movies end with sequel potential in just this way; the hero (or anti-hero) prevailed, but at a cost, perhaps a standoff, and addressing this cost is the topic of the next movie.