From experience and with personal opinion, I believe you have the most fun in writing while you feel as though your work is reachng a high level of creativity. The writer will see innovation in his writing, will become excited in seeing it as so, and then will continue to write to furfill their feeling of excitement. I myself went through this process when writing my college essay. I wrote whatever I wanted and became so enthusiastic and excited that I saw a potential in the ending result, and worked towards it. My strive was to the point where I was using the best of my grammer, and spending a lot of time into the piece of literature.
Try to have Fun!