Fun to me is reading novels, or solving puzzles, or doing unit tests, or going to Project Euler and trying to solve as many conundrums as possible with 8 cups of strong coffee and 5 hours in the dark of night. Fun to me is listening to jazz while sitting with a calculus textbook, trying to learn integration, or spending long hours on Wikipedia reading up about category theory after getting obsessed with Haskell.
Am I weird? Probably.