Most literal:
* Le cœur joyeux (formal)
* Le cœur heureux (formal)
More poetic:
* Le cœur joyeux (can be formal or informal depending on context)
* Le cœur qui chante (the heart that sings)
* Le cœur léger (the light heart)
* Le cœur rempli de joie (the heart filled with joy)
For a specific person's heart:
* Son cœur joyeux (his/her happy heart)
In a title or literary context:
* Le Cœur Heureux (The Happy Heart)
* Le Cœur Joyeux (The Joyful Heart)
Ultimately, the best translation will depend on the specific context and the desired effect.