In your dreams
Write a poem passing on wisdom about people to avoid, even if that person is you (;
Or perhaps an awkward situation
maybe even a choice you made that has caused a negative affect on your life
Heck it can even be some thing you thought was bad, but was for your own good.
The only theme I require is that
1.) you don't let the reader in on it until the end of the poem.
2.) You hand down some wisdom
3.) Use something in nature as a theme or a single metaphore
-"So you found a girl
Who thinks really deep thoughts
What's so amazing about really deep thoughts " Silent all these Years, Tori Amos