The only rule in today’s prompt is that the hobby may not be “writing”.
I have my own special reasons for this — namely: that, as an adult, I cringe every time I see a book where the main character is any type of writer. It seems to betray a lack of imagination. (Of course I’ll make an exception when re-reading books by LM Alcott or LM Montgomery or some other beloved writers whose initials are not “LM”, but for today the rule stands).
The hobby does not have to be anything you have done recently or frequently. It could be basket-weaving or finger-painting. But it should be something of which you have real-world experience and so can describe in minute detail if you need to.