Rules:
1. Let's use the three-line standard, with 5 syllables, 7 syllables, and 5 syllables.
2. Don't write the word "fall" or "autumn" as a season (leaves could fall, though).
3. Let's add this: If you want, make your poem three complete haikus -- each with a complete thought that all together address the fall season.
Crisp and dry, leaves cling
To weary, wrinkled branches
Then wave their good-byes.