Informal and affectionate:
* Qu'est-ce que tu fais, mon amour? (Most common and natural)
* Tu fais quoi, mon cœur? (Slightly more casual)
* Tu fais quoi, ma chérie? (Affectionate, but can be a bit more familiar)
More formal:
* Que faites-vous, mon amour? (More formal, but still affectionate)
Adding context:
You can also add context to make the phrase more specific. For example:
* Qu'est-ce que tu fais ce soir, mon amour? (What are you doing tonight, my love?)
* Qu'est-ce que tu fais de beau, mon cœur? (What are you doing that's nice, my love?)
Ultimately, the best way to say "What are you doing, my love?" in French will depend on your relationship with the person and the tone you want to convey.