One of these would do:
I sing in my room in my spare/leisure/in my free time.
I like to sing in my roon in my room in my spare/leisure/free time.
Singing in my room is my hobby/fave (favorite/preferred) hobby* in my free time.
But certainly there are other ways, as English always have multiple ways to express something.
"Hobby" could be replaced by "activity", etc...