To answer that question, I need to consider the meaning of joy. It is much deeper than fun and happiness, but we should never play down fun and happiness. We have enough misery to deal with π. Provided we aren’t doing anything wrong or hurtful to others or ourselves, fun and happiness are to be cherished as gifts from God. But joy goes deeper still.
Fun
Activities like playing games, watching a comedy, or going on an adventure. Fun is temporary and can be spontaneous.
Happiness
Influenced by life satisfaction, relationships, and personal fulfillment rather than just enjoyable activities.
Joy
Deeper and more enduring. It often comes from within and is tied to a sense of purpose, meaning, or connection. It can be present even in difficult times.

Joy can come from different sources, but for me, the one source that I wouldn’t want to be without is God himself. So, the one simple thing I do is worship God.
Here are two verses on joy, one from the Old Testament, the other from the New Testament:
Psalm 16:11
“You make known to me the path of life; you will fill me with joy in your presence, with eternal pleasures at your right hand.”
1 Peter 1:8-9
8 Though you have not seen him, you love him; and even though you do not see him now, you believe in him and are filled with an inexpressible and glorious joy, 9 for you are receiving the end result of your faith, the salvation of your souls.
Here are some of my favourite worship songs, all on the theme of joy. I’ve included Bach’s Jesu Joy of Man’s Desiring. I first heard that tune as part of Lady Lynda by the Beach Boys back in the summer of 1979. Then, in 1984, I played it on the organ for a wedding in Cork Baptist Church. It isn’t one I’ve ever sung. I can’t even say that I understand the lyrics, but I love the title, and the music.
Another classical hymn I’ve come to love in recent years is How Shall I Sing That Majesty by Libera. It doesn’t mention joy, but it fills me with joy to hear it.
How great a being, which doth all beings keep
Thy knowledge the only line to sound so vast a deep
Thou art a sea without a shore, a sun without a sphere
Thy time is now and evermore, Thy place is everywhere
How shall I sing that Majesty which angels do admire?
Let dust in dust and silence lie
Sing, sing, ye heavenly choir
Enlighten with faith my heart, inflame it with love's fire
Then shall I bear a part with that celestial choir
