Aside from child-care (which doesn't include occasional baby-sitting), medical, dental (includes non-cosmetic orthodontics) and vision (corrective visual aids -- okay, spectacles!) when necessary, the definition of S.7 extraordinary expenses are as varied as there are people.
Example: Child wants to play hockey. One parent can't afford the entire cost (which I understand can cost thousands of dollars a year), so request financial help from the other parent. The other parent, for reasons known only to him/herself, refuses. Given the cost of taking it to court, many single parents just give up and tell child there isn't money to play hockey.
It ISN'T about what the parent(s) want, it's about helping the child realize their potential and pursue their interests. Even with married parents both working there may not be enough left over for one child to play hockey or pursue whatever the child is passionate about.
Nor is it about one parent signing up the child for what the parent wants the child to take.
My ex refused/refuses to pay for any extra-curricular activities (swimming lessons), contrary to the agreement (which has become a court order), so I paid for them myself until I could no longer afford them. So the kids are denied the opportunities.
"If the parties cannot agree, is it the Judge who decides what is and is not a qualified expense?" Yes, that's one of the reasons family court thrives.
Gym membership at the Y? Maybe, but probably not
Private guitar lessons? debateable
Special school trips (In excess of $100)? I think so, though I'd increase it to over $250, but I'm not a lawyer or Judge
Photography classes? debateable
School Fees? generally, no (unless they are really high, like for specialized programs as IB)
Kickboxing Membership? debateable