* Buenos días - This is the most common and formal way to say "good morning" in any Spanish-speaking country, including Cuba.
* ¡Buenos días! - This is the same as above, but with an exclamation mark, adding a bit more emphasis.
* ¡Buen día! - This is a more informal, but still polite way to say "good morning" and is frequently used in Cuba.
You can choose the greeting that best suits the situation and the person you are addressing.