I don't do it anymore, but when I was in high school I absolutely LOVED writing Doctor Who fan fiction. Just basic short stories mostly, pretty much all centered around the 1st and 4th Doctors. Since they were my two favourite incarnations. Although, sometimes I would deviate from writing just basic short stories and try my hand at something a little more bold and original. Often with some rather cringe worthy results...
For instance, I once wrote this extremely awful and embarrassing "sequel" to Classic Who centered around Susan's daughter, Leah, (who was Half-Human and Half-Time Lord) carrying on her great grandfather's legacy by travelling across the universe in his TARDIS (which the Doctor had left to Susan as part of his will before succumbing to old age in his 13th incarnation). Susan had kept the TARDIS hidden away from her daughter in a secret room so she would have no idea of it's existence. But one day, Leah and two of her high school buddies discovered the TARDIS and managed to open it with the key they found earlier. Whilst marvelling at the TARDIS interior, one of her clumsy friends accidentally knocks a few switches on the console which causes the ship to dematerialise and whisk them off to parts unknown. Of course, since none of them have any idea how to pilot the ship they just keep drifting aimlessly from place to place. With nothing to help them except a few vague notes scattered around the TARDIS telling them how to operate the ship.
99% of the crap I made was incredibly stupid and poorly written, but still extremely fun for me to write nonetheless.