Fun is

-watching people walk the wrong way onto escalators and seeing their embarrassed faces.
-watching the grimaces and expressions of pain as you watch people climb stairs after a marathon and then realizing that you actually have to do the same.
-remembering the feeling of jumping out of a plane and falling and grinning yourself silly when you think about it.
-looking at the sunsets in Singapore and remembering that even in an urban jungle, you can see the beauty of nature as well.
-eating nutella + peanut butter sandwiches.
-spiking that volleyball after countless attempts.
-seeing the priceless expression on your friend's face when he hits the post when playing "inbetween" despite having an ace and a king.
-drawing a smiley face on a fogged up mirror or a dirty piece of glass.