1. Are you busy?

A: Hey, what are you doing?

B: Nothing, why?

A: Do you want to come get ice cream with me and Nina?

B: Sure! Where should I meet you?

In this dialogue, A is asking B what he is doing in order to make sure she can make plans with him. In this kind of situation, you could use these alternatives:
What are you up to (right now)?
Are you doing anything (right now)?
Are you busy (right now)?
Are you free (right now)?
Do you have any spare time (right now)?
Do you happen to have any spare time (right now)?

A: Hey, are you doing anything right now?

B: Not really, why? (Notice how the response changes here)

A: Do you want to come get ice cream with me and Nina?

B: Sure! Where should I meet you?

2. What is happening?

(Vivian walks into a smoky kitchen and sees Jeff burning food on a pan)

Vivian: Jeff, what are you doing?

Jeff: Trying to cook. I don’t think it’s working so well.

Here, Vivian is asking what Jeff is doing out of shock; she can’t believe what’s happening in front of her. If you’re in Vivian’s position, here are some other reactions you can use:
What’s going on?
What’s going on (in) here?
What is happening (right now)?
Since this is a strong response, you could also add some phrases like in the world or the hell to these expressions.
What the heck are you doing?
What the hell is going on?
What in the world is happening?

Vivian: Jeff, what is going on in here?

Jeff: I’m trying to cook. I don’t think it’s working so well.

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