"¡Qué césped tan bonito!"
Translation:What pretty grass!
As an agrostologist (grass scientist), I can tell you there are definitely some charismatic grasses out there, regardless of how high or not high I happen to be at the time.
Duolingo often seems to use bonito/pretty in situations where it is a bit odd. Pretty grass is yet another example.
I'm not sure it should be accepted. The word UN (a) isn't there, and bonito is pretty, not nice.