What metaphor or phrase can describe an object that is aesthetically pleasing yet totally useless?
Solution 1:
I would simply go with the word ornament. Decorative and serving no real purpose otherwise.
Solution 2:
Consider ornament, which principally means:
(n) a thing used to make something look more attractive but usually having no practical purpose
From: New Oxford American Dictionary
An object that's aesthetically pleasing and completely useless can be described as ornamental.