Word for "just for show"

"Ostensibly" is a good choice.

From Oxford Dictionaries online: "Apparently or purportedly, but perhaps not actually"

[http://www.oxforddictionaries.com/us/definition/american_english/ostensibly]

I believe there is no single word meaning "saying or doing something just for the show".

If you're looking for a noun defining the person who does it, I suggest

• "a phoney" - an insincere or pretentious person (TFD)

• "a beguiler" - someone who leads you to believe something that is not true (TFD)

• "a pseudo (noun)" - a pseudo-scholar, a pseudo-intellectual. (a pseudo-intellectual would be someone who pretends to know more than he actually does and exhibits memorized information ostentatiously)

If you're into urban language at all, poser fits the bill.I believe you will also find it in several dictionaries defined as such.

There is a term that comes close to this meaning:

"Face value"

If something is taken at face value then it is evaluated based on its outward appearance.

If you are looking for a more literal meaning:

"Decoration"

or

"Ornament"

A decortaion by its very definition is "just for show" and an ornament is specifically not supposed to be used, existing again "only for show"

If you mean the ideal of being showy but not having any depth, that is a person who care more for appearances than real things, then suitable words would be:

"Shallow", "Pretentious", "Fake" or "Image obsessed".