Fun was a concept.
When asked for his name, rank, and intention at Bowie Base One, the Tenth Doctor responded, "The Doctor, doctor, fun." (TV: The Waters of Mars [+]Loading...["The Waters of Mars (TV story)"])
The Eleventh Doctor once wrote in his diary that from 10am - 11am, he watched The Story of Tricey Beaker and Bear With Me on TV, which he described as "fun". (PROSE: The Doctor's Diary [+]Loading...{"page":"15","1":"The Doctor's Diary (DWA 308 short story)"})
Behind the scenes
In non-valid sources
After tricking the Tenth Doctor into thinking he'd returned his DVD copy of Schwarzenegger to Arnie's Videos 400 years late, and demanding four million pounds, leading the Doctor to disappear, Schwarzenegger reflected that the Doctor hadn't been late in the first place, and that he loved having fun with "those crazy Time Lords". (TV: Series 1 Episode 6 [+]Loading...["Series 1 Episode 6 (Hedz TV story)"])