Formal:
* ¿Qué está haciendo? (This is the most formal way to say it.)
Informal:
* ¿Qué haces? (This is the most common and informal way to say it.)
* ¿Qué andas haciendo? (This is a more casual way to say it.)
* ¿Qué onda? (This is a very informal and slang way to say "What's up?" or "What's going on?")
Example:
* Formal: ¿Qué está haciendo usted? (What are you doing?)
* Informal: ¿Qué haces, Juan? (What are you doing, Juan?)
* Informal: ¿Qué andas haciendo por aquí? (What are you doing around here?)
* Very informal: ¿Qué onda, amigo? (What's up, buddy?)
Remember that the best way to choose the right translation depends on who you're talking to and the situation.