A hobby was an activity that one engaged in for fun.

Trainspotting was a hobby that the Doctor had participated in at some point during his long life, but which he was "rather rusty" at by his sixth incarnation. (AUDIO: The Nowhere Place) As well as having an interest in real-life trains, the Doctor also enjoyed using a model train set, and added to his collection of model trains over many incarnations. (PROSE: Model Train Set)

Robin Hood believed that "everyone should have a hobby", and claimed that his was annoying the Sheriff of Nottingham. (TV: Robot of Sherwood)

Clara Oswald identified travelling with the Doctor as her hobby. (TV: Into the Dalek, The Girl Who Died)