当被问到“干什么”时，可以用以下几种英语表达：

What are you doing？

What are you up to？

What is he doing？ （如果是指别人）

What do you do？ （更随意的问法）

I'm just doing some random things.

Not much, just hanging out.

I'm just killing time.

Just chilling out, you know.

Just doing some routine things.

I'm working from home.

I'm cooking a meal.

I'm watching a movie/TV show.

I'm reading a book.

I'm exercising.

I'm spending time with my... （可以接任何人称）

根据具体的语境和对话的亲密程度，可以选择合适的表达方式。例如，在和朋友闲聊时，可能会选择更随意的表达，如“What are you up to？”或“Just chilling out, you know.”；在工作场合，则可能会更正式地回答“I'm working from home.”或“I'm cooking a meal.”。