Just about anything fun on the C64 normally required a ton of "pokes" and "peeks". Besides obvious instructions like "border" and "background", I created instructions to handle sprites, sound, and graphics. The graphics included drawing of lines, circles, fills, rects, etc. The C64 didn't provide any of the functionality, so I had to figure out efficient ways to draw lines. No big deal, but still, back then, this was a big deal.
More sophisticated functions included "asin", "acos", "atan", and drawing 3D meshes.