Unhappier, as the word suggests, means less happy or less content. The antonyms of unhappier, then, would refer to the opposite: increased happiness or contentment. Synonyms for happier might include joyful, elated, pleased, content, satisfied, and fulfilled. The antonyms could also be phrases, such as "more at peace" or "finding tranquility." Unhappier might indicate a range of negative emotions from mild to severe, so the antonyms would reflect the spectrum of positive emotions, from a feeling of calm to exultation. Essentially, any emotion that represents satisfaction or pleasure could be an antonym for unhappier.