Your topic is the end of summer (or the beginning of fall).
Your form is: carmina figurata
Here’s and example, Robert Herrick’s The Pillar of Fame:
Fame’s pillar here, at last, we set
Out-during marble, brass, or jet,
Charm’d and enchanted so
As to withstand the blow
Nor shall the seas,
Of storms orebear
What we up-rear,
Tho Kingdoms fall,
This pillar never shall
Decline or waste at all;
But stand for ever by his owne
Firme and well fixt foundation.