Synonyms:
* Joyful
* Cheerful
* Delighted
* Glad
* Merry
* Elated
* Content
* Pleased
* Thrilled
* Ecstatic
* Blissful
Words that convey similar feelings:
* Optimistic
* Upbeat
* Positive
* Enthusiastic
* Excited
* Lighthearted
* Carefree
Words that describe the outward expression of happiness:
* Smiling
* Laughing
* Radiant
* Beaming
* Bubbly
Words that describe the state of being happy:
* Serene
* Peaceful
* Relaxed
* Fulfilled
* Grateful
Words that describe the opposite of happiness:
* Sad
* Depressed
* Unhappy
* Miserable
* Melancholy
Figurative language:
* On cloud nine
* Walking on sunshine
* Over the moon
* In seventh heaven
This is just a starting point. The best word to describe "happy" depends on the specific context and the nuance you want to convey.