A really fast thesaurus... with lots of synonyms.
Words to describe a smile include warm, beaming, faint, mischievous, and radiant — pick one for the warmth, the size, the subtlety, or the mood behind it.
Describe the warmth (warm, tender, genuine), the size (beaming, broad, faint), the mood (wistful, joyful), or the intent (mischievous, sly, knowing) — each captures a different kind of smile.
A happy smile is often beaming, broad, radiant, or gleaming — words that suggest an open, unmistakable show of delight rather than a small or guarded one.