Directly related:
* Happiness: The state of being happy.
* Happily: In a happy way.
* Happy-go-lucky: Carefree and lighthearted.
Less direct, but related in meaning:
* Joyful: Filled with joy and happiness.
* Cheerful: Happy and optimistic.
* Merry: Full of gaiety and happiness.
* Glad: Feeling happy and pleased.
* Delighted: Feeling great pleasure or happiness.
* Content: Feeling satisfied and happy.
* Blissful: Extremely happy and peaceful.
* Exuberant: Full of energy and enthusiasm.
* Elated: Feeling very happy and excited.
Words related to the opposite of happy:
* Unhappy: Not happy.
* Sad: Feeling unhappy or sorrowful.
* Miserable: Very unhappy or uncomfortable.
* Depressed: Feeling very sad and hopeless.
These are just a few examples, and there are many other words that could be considered related to "happy" depending on the context.