She had a downcast (or dejected) look on her face.
Even unsmilingly can convey different emotions: sadness, anger, even no expression at all.
If it’s the opposite of happy you want, you may be stuck with emotions instead of expressions.
yDc’s Thesaurus gives unhappy and disconsolate as the antonyms of happy. It gives the antonym of smile as frown, which of course may not convey sadness.
She had a doleful/dolorous or cheerless look upon her face.
Tough assignment!
I may not have this quote exactly, but it fits this situation:
"The difference between the right word and the almost right word is like the difference between lightning and the lightning bug.
—Mark Twain