What metaphor or phrase can describe an object that is aesthetically pleasing yet totally useless?

I would simply go with the word ornament. Decorative and serving no real purpose otherwise.

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.

Eye candy: "a slang term for attractiveness"

(US, idiomatic) Any object or sight with considerable visual appeal. The computer graphics added lots of eye candy to that movie.

Of a person

Bimbo

Webster's: "an attractive but stupid woman"

Beautiful fool

To my knowledge this is a reference to The Great Gatsby meaning a woman who is pretty yet blissfully ignorant. Perhaps it predates that?

Plastic

Webster's: "not real or sincere" [i.e., superficial, malleable]

Arm candy

Webster's: "a young attractive person who accompanies a usually older person at social events"

Trophy Wife/Husband

Slang: a spouse who is valued for superficial reasons.

Of an object

Bauble

Webster's: "something of trifling appeal" [lacking in significance or solid worth, frivolous]

Trinket

Webster's: "a piece of jewelry or an ornament that has little value" [therefore shiny, aesthetically pleasing... not useful]

Ornament

Webster's: "something that lends grace or beauty" [Webster's example sentence reads: "The columns are there purely as ornament—they have no structural function."]

Tchotchke

Dictionary.com: Syn for Oranment, Trinket

Clearly these aren't all perfect fits as they stand by themselves, but given the proper context they could all be used to convey a sense of useless splendor.

Edit: I removed a couple that would have required more finesse to work into the context.