As a fellow Maths fanatic I would suggest 'The story of Maths'. It teaches you the history of it and where some of the most basic principles come from. But this wont really help your application much, alot of people will watch this who are interested in Maths.
Olympiads would be one of the biggest things that can set you aside from the rest or any other national tests that challenge the mathematically gifted.
I'm thinking of creating android apps and small games/puzzles, I might then create a website and upload them all on to that, so the University can view them. Thats if I choose to study Computer Science at University and not Physics...