Photography.
But the line is so blurred these days i am not sure if it is still a hobby or a job.
I still shoot for fun but mostly I shoot for clients of late.
Or if you are not a retard, Photography, another one that I dont think has been mentioned... Home improvements/decorating (if you have your own place) very time consuming and you will have a sense of accheivement once you are finnished, then start again
Learn a language. Spoken or computer.
Mine is 3D printers, I have 2 of them now.
This is my first one, it's had a lot of upgrade since I shot this video: