It can be and I ever thought when I'm free sometimes thinking of things you did it that can make useful to you. It's like review what you did and may be make you know It 's good or bad for preparing to solve it or remember to not do that again and being with yourself make you know oneself more than before in my thinking.
I dont think so if it is a hobby. A human brain is busy in thoughts all the time and it cant stop thinking. So I dont think so if it can be called a hobby however it is nature.