An essay that long based on a word which involves the word "Smile" , better put on your thinking cap and get that imagination pumped up !
First, you might introduce your essay with a quote on happiness from a famous author or whoever that comes into your mind.
Then, write about the introduction on happiness. Give the definition on the word, what does it really imply? What is your reaction when that word pops into your mind? Filled with endless joy? Or nothing at all?
Next, just imagine : What is the world when happiness is around? Will they live in harmony or will they slit one's throat and peace will never be an option? Explain your thoughts further and let bombastic vocabulary and creative writing rule you.
Lastly, write your conclusion or hopes on the word.
Or you can write a story on people like you, was saved because of being happy.
Hope this helps.