No idea what you would put in an academic one
But for sports, it would be description of what sports they play, and at what level (supported by references from clubs), how long they've been doing it, any development pathways they are on, notable achievements (not just competition success but things like early selection for an A team, season 'best player' or 'best attitude' award etc, any refereeing coaching, residential camps?)
And let a love of being active shine through - so not just organised sport, but things they do on holiday. Yes, you need to show talent/achievement in a main sport or two. But those who go on to have high level achievement in sports in their later teens are (with very few exceptions, notably gymnastics) active in a number of sports at that age - so kayaking on holiday, expeditions on bikes, parkrunning, dance, experiments with archery, sailing etc could all be relevant.
You want to show that they are achieving, have a good level of all round fitness, love sport and being active and respond well to being coached
I guess it would be similar for academics - show love of learning by enrichment activities, including things like activity days at museums, online challenges (things like Smallpeice for children's STEM challenges) creative writing competitions, UKMT junior maths challenge, clubs like Young Archaelogists ???