A good Frost quick-hitter is

  Nothing Gold Can Stay

  Nature’s first green is gold,
  Her hardest hue to hold.
  Her early leaf’s a flower;
  But only so an hour.
  Then leaf subsides to leaf.
  So Eden sank to grief,
  So dawn goes down to day.
  Nothing gold can stay.

EDIT: If you're looking for one to memorize, I should say.