There is no trick to this question. Usually when poeple ask you "what you do for fun?" they generally want to know what your hobbies are. They want to know if you have any social lifestyle that is compatible with theirs. I ask girls this question most of the time. When they respond:
"I like rock climbing, going to the movies (with my friends)", etc, I say "No way, I just got back from Corolado climbing moutains this past weekend. It was awsome. Maybe we should get together sometime and do some serious mountain climbing" or when they response with watch movies, I go.." Really, what was the last movie you saw with your friend AT THE THEATRE?, don't tell me Men In Black".
All that I'm saying is to don't sweat it when asked that question. Personally I won't tell any woman I don't do anything for fun! Make up something, even if you don't have anything. They do it all the time. If you have enough confidence, you can respond " I am booked for Fridays. If it is the dinner that you're trying to take me to, we can do it any other time except on fridays...."