The antonym is concerned to indicate just it opposite of the meaning of a Word. In this sad case is an augmentative of the sad Word then the antonym of sad would be super happy, cheerful, encantadisimo.
Only to complement the response above, the superlative of happy is felicísimo and of cheerful is alegrisimo. Another antonym more could be thrilled.
Lively, hilarious, achispadisimo, excitadisimo
Antonyms of sad, serious: alegrisimo, thrilled