I encountered this when I was watching a Black Diamond Equipement video about two guys trying to free a new line on El Cap. It's a pretty simple but sensible scale of how fun the activity your doing is. It's simply seperated down into types 1, 2 & 3 fun:
Type 1 fun:
Something that had like instant gratification, somewhere that you can enjoy in the moment. Examples are good sex, good food, good climbing you get the idea. This is what your aiming for in an activity.
Type 2 fun:
Something that you look back on and think "hey, that was fun" but didn't actually seem like it at the time. Also could be called "retrospective fun". Examples could be when you've done a route that scared the sh*t out of you, but wasn't actaully that bad. You'd look back on the shared experience you and your partner had, thinking it was fun. For me, hanging belays just above the ocean are probably type 2 fun.
Type 3 fun:
Something that isn't fun at all, not even when you look back at it and you remember at the time screaming at yourself (usually in your head) as to "why the bloody hell are you doing this?". It's the sort of thing that you tell your friends "If I ever have an idea as stupid as that again, slap and don't let me do it". Strangly all of the thing that I've done tha are type 3 fun may not have been fun, but definatly left a strong memory or impression on me.
I guess the important thing to take from this is that, it doesn't have to be fun to be fun right?