little, lucrative, nice, profitable, useful (all esp. BrE)
He decided to turn his hobby into a lucrative sideline.
He had found himself a nice little sideline painting windows.

have
develop

sideline for
Cider-making was a sideline for many farmers.

sideline in
She is developing a nice little sideline in babysitting for friends.

sideline to
The restaurant started out as a sideline to his main business.

Sideline is used with these nouns as the subject:
injury