Type I fun, F I:
It's fun when you're looking forward to it, it's fun when you're doing it, it's fun when you're thinking back on it. It's fun that anyone can appreciate, young or old.
Examples: skiing, mountain biking, hiking in the autumn.

Type II fun, F II:
It seems like a good idea beforehand, but it's sort of rotten when you're in the thick of an F II experience. By the time you're in the car driving home, warm and fed,
you realize it was actually a pretty good time, usually due to having good company at the time. Think low grade epics in non-lethal, scenic terrain.

Type III fun, F III:
Obviously not a good idea, unconditionally miserable during the experience. F III's are still not fun on the drive home, but after a couple days
you realize it was still better than going to work in the morning.

Type IV fun, F IV:
Clearly awful in every way--but at least it makes a good story! Epics through and through, usually near-death in some respect.