In terms of 'forked out', money I have viewed as a special expenditure in addition to the usual clothes, food, school equipment, travel, etc, I would include:
Musical instruments. DS is enthusiastic and accomplished, and his playing is part of his school life, academic achievement and increasingly, social life. he has not demanded or expected new instruments, he has saved diligently for upgrades and different models - but twice we have invested what we would consider 'forking out' sums to top up (double)his savings, and bought one outright for a birthday.
Theatre tickets for plays that are on the school curriculum, and shows like War Horse and Curious Incident
I once took DC to CERN for the day - return tix on EasyJet cost us less than £60 between us and it was a fabulous inspiring day.
'Added value' in terms of experiences. We are not ones for spending £££ at theme parks etc, but when on a day out or holiday will spend to make an experience really worthwhile. High spends on specific outward bound type activities and course, a flight in a small plane, hiring boats etc - fun and an 'experience'. We tend to holiday in quite modest ways, no hotels or Florida, and we don't tend to seek out 'attractions' for days out. I have been spared, for example, Peppa Pig World, but we did go to Port Lympne and Howletts - big entry prices.
We spent a lot of Scout trips and expeditions - they went on 6 or 8 one summer, at £25-40 a time, but I found that money well spent, for overnight / weekend trips.
We go camping , fell walking, visit parks and walk, NT properties. I never spend vast amounts feeding us al at fast food and overpriced 'attraction' places.
We spend no money (and are not asked for it) for designer or branded clothes - DC and friends consider it a rip off.
I think I am lucky in not being badgered for money for expensive events. I feel I am getting off quite lightly in these teen years!
I know there is a school field trip on the horizon that will cost £600 so we are saving for it.