What are you doing?
I'm swimming.
What are you doing?
I'm playing soccer / football.
What are you doing?
I'm playing basketball.
What are you doing?
I'm playing baseball.
What are you doing?
I'm doing karate.
What are you doing?
I'm playing volleyball.
What are you doing?
I'm skiing.
What are you doing?
I'm playing tennis.
What are you doing?
I'm playing badminton.
What are you doing?
I'm running.