seems like some of the above posters enjoy arguing and judging/slandering each other on forums, a mindless hobby. I dont recommend that.
I can recommend origami
or crossword puzzles.
jigsaw puzzles - these dont need to be expensive if you get them from the op shop, do them take a photo once you are done then pass them to someone else
if you like numbers...sudoko
some people like birdwatching or trainspotting. Basically what you do is identify as many birds as possible or trains. The thrill is catching them on time. If youve got a camera you can take photos, or just record them in a notebook. Then you can join a club where everyone is a trainspotter or birdwatcher and you can say I saw so and so train or I know xxx number of birds.
Sounds daft but actually lots of people do this! It could be anything doesnt have to be birds or trains.
You could be a mystery worshipper...attend as many church services as possible as a visitor and record what songs get sung or sermons are preached in each of them.