Poem: No. Unless it's funny. One month isn't anything to get all sappy and lovey dovey about. Sure, make a deal about it, but it shouldn't be a BIG deal.
Take her out somewhere different.
If she goes to your school, walk up, give her a random kiss, and walk off. If she asks why later on, tell her. Something small, and sort of sappy, but not creepy.
My favorite. Buy her a congratulations card.
Write
"Congratulations babe. You've lasted a whole month with me. You should feel blessed
"