* διασκέδαση (diaskedasi): This is probably the closest general equivalent and refers to amusement, entertainment, or recreation. It's a good choice for many contexts.
* ηδονή (ēdonē): This word means pleasure or delight. It's suitable if the "fun" involves sensual pleasure.
* χαρά (chará): This means joy or happiness. It's appropriate if the fun is associated with a feeling of great happiness.
* παιχνίδι (paichnidi): This means game or play. It's suitable if the fun involves games or playful activities.
The best word to use will depend heavily on the context.