When I was in my final year of HS, me and one of my mates in the advanced math class programmed a Mandelbrot set generator.
That really made my mind race ... the realisation that simple rules can yield infinite variety.
We managed it in an afternoon, admittedly we were both already proficient programmers, but I will always remember that afternoon, and Mrs Munn our maths teacher who helped us (thanks Mrs Munn, you rule).