Subjectivity of having Serious Fun
The word fun is a very subjective term and people have very different notions of what fun is.
For some having fun means going out and partying with friends, for others it means hitting the ski slopes, and for others it means sitting in front of the computer playing games for hours on end. If a person enjoys an activity then that activity is fun for that person. However, the same “fun activity” can be deemed by another person to be either boring or even scary.
Having serious fun is, of course, just as subjective, if not more so. It indicates a greater degree of enjoyment. To an adventurous person riding a motorcycle might be fun but going sky diving is serious fun. To a partygoer, Friday nights can be fun but spring break means serious fun. To a shopaholic buying a pair of jeans is fun but going on a shopping spree is even better.
Of course, being a very subjective thing, the degree at which one enjoys an activity differs for each person so that someone might need to buy a closet-full of new clothes to call it a shopping spree while another person who just bought a whole new set of clothes might consider that already a shopping spree.
The same goes for partygoers, someone might consider having several bottles of beer and dancing for hours on end serious fun while another might only consider it serious fun if he/she gets drunk.