A friend of mine practices two hobbies: photography and tennis. 

He recently found a way to blend them together by volunteering to take photos of local tennis clubs and tournaments.

He could probably get paid a small fee, of course, but then he would be taking on a client. He would feel obligated to take on their suggestions.

Instead, he’s giving his time and energy as a gift. In exchange, he gets the freedom to practice his craft how he likes.