Formal:
* आप क्या कर रहे हैं? (aap kya kar rahe hain?) - This is the most formal way to ask the question, appropriate for use with elders or someone you don't know well.
Informal:
* तुम क्या कर रहे हो? (tum kya kar rahe ho?) - This is a more informal version, suitable for friends or family.
* क्या कर रहे हो? (kya kar rahe ho?) - This is even more informal and can be used with close friends.
Other options:
* क्या चल रहा है? (kya chal raha hai?) - This is a more casual way to ask "what's up?" or "how's it going?"
* क्या कर रहे हो? (kya kar rahe ho?) - This is a more general way to ask "what are you doing?", but can also be used to ask "what are you up to?"
The best translation for you will depend on your specific context and your relationship with the person you are talking to.